PRINCETON, N.J.--The Princeton University judicial committee Wednesday night issued "warnings"--the lightest punishment possible--to 72 students who held an all-night sit-in March 17 in Firestone Library to protest Princeton's holdings in corporations doing business with South Africa.

The university has taken no action against either Oppit Webster, a 1979 graduate and an alumni trustee, who stayed with the protesters in the library all night, or five professors and a university chaplain, who began the sit-in but left shortly after midnight after heated discussions with administrators.

A spokesman for the People's Front for the Liberation of Southern Africa, which sponsored the sit-in, said Wednesday although she was pleased the punishment had been light, the students should not have been disciplined at all.

At the time of the sit-in, the Front announced that Princeton had not publicly revealed that part of the endowment of the Woodrow Wilson School of Public and International Affairs is invested in corporations doing business with South Africa.

Portfolio Disclosed

Two weeks later the administration disclosed the portfolio of the Woodrow Wilson School, and the report showed that about 30 per cent of the $47 million endowment is invested in firms active in South Africa.

A university spokesman said 12 of the 16 companies in the portfolio have pledged to comply with the Sullivan Principles, a set of guidelines for corporations in South Africa.

Members of the People's Front boycotted classes yesterday, on the 12th anniversary of the assassination of Martin Luther King Jr. About 200 students attended a noon rally but students contacted yesterday said class attendence was not markedly below normal.

Instead of attending classes, Front supporters attended chapel services for King, seminars, and other activities.