Herbert A. Philbrick, for nine years an undercover agent for the F.B.I., yesterday began an expose of communist infiltration in the Cambridge Youth Council, a "pacifist" organization which in 1940 drew much of its support from Harvard and Radcliffe.

Writing the second of a series of 17 articles entitled "I Led Three Lives," appearing in the New York Herald Tribune, the former "counterspy" declared that the "apparently altruistic youth organization" was actually a communist front. The articles are condensations of his book which will be published by McGraw-Hill next month.

Philbrick, a Boston advertising man, was a key witness at the trial of the 11 top communists in 1949. His appearance in court was in effect the first disclosure by the federal government that it had undercover agents in the Communist Party. He is also the accuser of Dirk Struik, suspended M.I.T. professor.

In the article, Philbrick explained how he became chairman of the Youth Council which he innocently believed to be a valid "pacifist" organization. Its organizer, said the counterspy, was a girl named Toni Grose who had her offices in the Freshman Union. Its list of prominent sponsors included Kirtley F. Mather, professor of Geology.

He soon realized that he "had walked into a cleverly laid trap" and became an agent for the F.B.I. in order to make up for his mistake.

Appearing with Philbrick's article are several letters written while he was chairman of the Cambridge Youth Council. One of the letters is signed by Struik.