ACROSS Milk St. from the Post Office Building where Dr. Benjamin Spock's case is being heard stands a sign that reads in large green letters: "Boston Five." A second look reveals smaller letters that complete the title of a branch office of The Boston Five Cents Savings Bank. But the anonymous adman who decided to capitalize those two particular words inadvertantly provided a sort of marquee for the drama of the so-called Boston Five and their fight against government prosecution for illegally counseling draft resistance.

Inside the Post Office, in an austerely decorated twelfth-story courtroom, the adversaries in the case gathered last week for the first encounter in what may be a long legal duel. The five defendants--Spock, Yale Chaplain William Sloan Coffin, Harvard graduate student Michael K. Ferber, writer Mitchell Goodman, and former National Security Council staffer Marcus Raskin--were all there, each with one or more attorneys. So were Judge Francis J.W. Ford, who will hear the case, and assistant U.S. attorney John Wall, who will argue the government's side, at least at first. In addition, there was the usual knot of reporters and a few miscellaneous onlookers, including Jessica Mitfrord, author of The American Way of Death, and Col. Paul W. Feeney, director of the Massachusetts Selective Service System. Although few spectators appeared, the horde of defendants, counsel, and relatives necessitated setting up a double row of extra chairs--and at least one lawyer still wound up in the jury box.

Ford's first words barred an entire area of potential defense material: the legality of the Vietnam war. Ford later elaborated his ruling to exclude from the trial both the Nuremberg principles and any attempt to question the validity of executive action taken without Congressional approval. The defense attorneys could not have been overly surprised, since American courts have always shunned issues of war and peace as being unsuited for judicial inquiry. In fact, sticking to more established lines of defense probably gives the five men a better chance of acquittal. But anyone who hoped Spock might be martyring himself to open a legal route of attack against the war is sure to be disappointed.

EVEN with the legality of the war set aside, the Spock case seems certain to raise fundamental questions of free speech and the limits of dissent. Last week's hearings, in theory, dealt only with several defense motions to dismiss the indictment as too vague, or to force the government to reveal more facts about its charges of conspiracy. In arguing the motions, however, the five's lawyers rehearsed a powerful defense based on the First Amendment's guarantees of free speech.

The defense's central point is that the five are charged merely with speaking against the war and the draft, and not with committing substantive crimes. "So long as they are discussing public matters, no matter how vigorously, and so long as they do not reach incitement (to break the law), they are protected," said William P. Homans Jr., Ferber's lawyer.

But there are two problems with the free speech agrument. Speech is not absolutely protected, and the man who, as in Justice Holmes' famous example, shouts fire in a crowded theatre is liable to prosecution. Spock and the four others are accused of interfering with the operation of the Selective Service System by verbally counselling young men to defy its regulations, and if the government can prove that their speech was the direct cause of someone's lawbreaking, they may well be convicted.

Second, the five are accused not only of speaking against the draft but also of collecting draft cards (on Oct. 16 at the Arlington St. Church) and turning them in to the Justice Department (on Oct. 20 in Washington). This, the government maintains, is clearly not speech but illegal action.

IN ANSWER to the first objection, the defense may claim, in Holmes' words, that "turning in draft cards was not a result of aiding, abetting, or counselling by any one of these defendants, but an act of individual conscience." The second snag is more difficult. The defense has said that a close reading of the Selective Service regulations shows that Congress never intended to make failure to carry your draft card with you illegal. In that case, collecting draft cards would be no crime at all. In addition, the five's lawyers contended during the hearings that turning in cards is a form of "symbolic speech" and thus as protected as a sermon or an address. But the concept of symbolic speech--at least to this date--has received a decidedly unsympathetic welcome in the courts.

Another defense argument, advanced by James D. St. Clair, Coffin's attorney, is that the value of encouraging as open a public discussion as possible under the First Amendment overbalances the relatively harmless violation of draft regulations. (St. Clair himself, a lecturer on law at Harvard, is a veteran of liberal campaigns. He was one of Joseph Welch's chief aides in the Army-McCarthy hearings of 1954.) To convict the five men, St. Clair said, would have the effect of "chilling" debate on the draft and the war by raising the menace of Federal prosecution.

In the hearings, however, free speech arguments remained mostly in the background as the defendants' lawyers assailed the indictment itself, handed down by a Grand Jury in January. The indictment declares that the five men took part in a nationwide program of draft resistance and the defense kept up a drumfire of demands that the government produce specific information to substantiate the existence of the anti-draft conspiracy. The lawyers pressed particularly to discredit the government's depiction of the alleged conspiracy's size. "Anything that happened within the nation--I would assume all 50 states--could come with in the purview of this indictment," St. Clair said. Leonard Boudin, Spock's sad-eyed attorney, was particularly critical of the indictment's supposed vagueness.

REPLYING for the government, Wall scoffed at the defense's insistence on knowing who the co-conspirators were and which men had been enticed into breaking the law. "All Mr. Boudin has to do is go to the Bureau of the Census and ask for the name of every male person between 18 and 35 and he'll know who's been counselled," Wall said.

The defense instantly pounced on Wall's words. Boudin called his description of the conspiracy "astounding." According to the government, Boudin said, "The co-conspirators consist of every person in the United States of draft age." Homans claimed that if the government was saying that every male of draft age in the country had been counselled to defy the law, then the indictment was clearly "overbroad."

Now Ford has the various defense motions under advisement. When he releases his rulings--which may not be for several days or several weeks--he should set a trial date. The chances are that the indictment will not be dismissed outright, and a full-scale trial will ensue. Even if Ford assiduously blocks every effort to slip the legality of the war into court, there are still enough basic issues of constitutional laws involved in the case to deserve headlines for a long time.