Tuesday night's blockade of a South African diplomat in the Science Center was not the first time Harvard students have expressed distaste for speakers on campus.

Twenty years ago, students outside of Leverett House blocked the car of Secretary of Defense Robert McNamara, who was in town for a speech. McNamara got out, debated students on the Vietnam War, and was allowed to drive on.

Since then, Harvard has seen a series of protests against politically conservative speakers.

In November of 1983, a speech by Secretary of Defense Caspar W. Weinberger '38 was interrupted repeatedly by shouts and chants from the audience. Members of the audience threw two balloons full of red dye at Weinberger, but the liquid missed the Secretary. Twelve students dressed in grim-reaper robes stood silently throughout the speech and pointed at Weinberger.

The hecklers drowned out Weinberger several times, prompting the executive committee of the Faculty to discuss the need to protect free speech more stringently on campus. Reviving the Vietnam-era Committee on Rights and Responsibilities (CRR) was considered, but nixed because the body had faced student boycott almost continuously since its 1969 inception.

But the Faculty did reconvene the CRR in May, 1985, after 10 years of dormancy, to hear the cases of 25 students involved in two divestment protests--a sit-in at the 17 Quincy St. headquarters of Harvard's governing Corporation and a blockade of South African General Consul Abe S. Hoppenstein in the Lowell House junior common room.

The Conservative Club had invited Hoppenstein to speak, but divestment activists formed a human blockade to prevent him from leaving Lowell House. Harvard police surrounded Hoppenstein and pushed through the protest.

The CRR gave 10 students suspended requirements to withdraw for their part in the Lowell blockade and 11 students admonishment for their role in the 17 Quincy St. sit-in. The CRR dropped charges against four of the students involved in the blockade.

Last spring, speeches by two Nicaraguan Contras were halted when members of the audience hurled eggs and a jar of fake blood at the two speakers invited by the Conservative and Republican Clubs. The demonstrators were not Harvard students.

Five weeks later, under extremely tight security, one of the Contras returned to Harvard. The College had extra policemen on hand, in addition to several videotape cameras trained on the audience. About 10 minutes into the speech, roughly one-third of the audience stood up and left in protest of the Contras.

In November of 1986, protesters screaming and writhing on the floor forced the relocation of a speech at the Law School by Philadelphia Mayor Wilson Goode. Six Harvard policemen escorted Goode out of the auditorium, and officials announced the new location for the speech. Officials excluded the protestors from the new auditorium, and Goode spoke uninterrupted.

Last month, the Conservative Club sponsored a speech by William K. Coors, chairman of Coors Brewing Co. Coors spoke to a generally respectful audience at the Science Center, while about 200 students and union workers protested outside for one-and-a-half hours.