South Africa is a pigmentocracy in which a white minority rules over a black majority by dint of sheer terror. Officials practice torture in South African jails, and scores of political prisoners have been killed while in police custody. Although blacks constitute about 70 percent of the population, they have no say in the government under which they suffer.

It is right and just for Americans to demonstrate their opposition to apartheid. But the justice of their cause does not exempt them from dilemmas involving the morality of political action. For instance, controversy has surrounded cases of collegiate anti-apartheid activists who disrupt pro-apartheid speeches by officials of the South African regime.

Such activists are certain to be condemned by reactionary bigots who embrace the idea of freedom of expression only when it can be used to whip leftish dissidents into line. But they will also likely be criticized by persons or organizations whose views on racial justice and civil liberties are entitled to respect; I think here particularly of the American Civil Liberties Union. All will resort to a set of precepts that exalt equal, neutral, symmetrical tolerance.

Many civil libertarians assert that all speakers invited to a campus must be allowed a hearing. Nat Hentoff implicitly adopts this position in a column {op-ed, June 6} in which he criticizes my contention that, in some circumstances, disruption of speech because of its content may be justified. The editors' headline on the column reads, "This Professor Says Free Speech is Fine -- Up to a Point," as if it is somehow deviant to believe that freedom of speech should have limits.

But even the ACLU recognizes that in any decent society freedom of speech must have some bounds. For without bounds, this freedom -- like any freedom -- can turn into an oppressive force. As President Derek Bok of Harvard University has observed, with respect to freedom of expression on campus, "free speech, though extremely important, must be fitted together with other rights and legitimate interests."

Civil libertarian absolutists such as Nat Hentoff fail to grapple with the complexities posed by clashing interests and instead seek security in automatic responses to abstract rules that disregard context. That is one reason why the contours of freedom of expression are undergoing spirited reappraisal from a wide array of perspectives. Conservatives such as George Will, feminists such as Catherine McKinnon and liberals such as Owen Fiss have become understandably dissatisfied with explanations that neither explain the actual regulation of speech in society nor generate an inspiring vision of better arrangements.

It won't do to say categorically that suppression based on the content of speech is always wrong; what about proscriptions against perjury, threats, obscenity, libel, "fighting words," misleading advertising, invasion of privacy, advertisements for employment that excludes women or minorities, advocacy directed to inciting imminent unlawful action? It won't do to say simply that actions silencing speech can be dangerous; in a given situation, failure to repress speech because of its content can also be dangerous. It won't do to say unequivocally that tolerance is a virtue; tolerance may merely signal fear, or callousness, or the smug confidence of the strong.

Appalled by my argument that suppression of inhumane expressive conduct is sometimes justified, Hentoff asks: "Would {Kennedy} approve his students' taking over his class if they found him 'inhumane'?" If in fact I did act in a sufficiently offensive manner, a student takeover of my class might be justified, although my position as a professor should give me extraordinary leeway. If I repeatedly stated in my course on contracts that Jews or blacks or Asian Americans are genetically incapable of trustworthy behavior, there would indeed come a point at which the students in the class could legitimately disrupt it. Ironically, while Hentoff accuses me of being suppressive, it is he who would apparently subject students, and society at large, to the tyranny of certain absolutely inflexible rules.

Because freedom of expression is such a rightly cherished aspect of academic life and democratic government, university authorities should view disruption of a speaker as presumptively illegitimate. But protesters seeking to justify disruption should be allowed to meet that burden of proof by drawing attention to relevant considerations including the harms that would have likely ensued from the speech, the degree to which the speaker has access to other forums of communication, and the manner in which the dissidents accomplished their goal. Freedom resides not only with the speaker but with protesters, the immediate audience, the university community, and those beyond -- including the blacks of South Africa.

Perhaps university authorities will not want to invest scarce resources in exhaustive case-by-case adjudication. In that event, they should make clear their rationale for disciplining disruption without pretending, as is often done, that suppressing speech is necessarily wrong.

Another concern is that even holding out the possibility of exoneration will accentuate the risk of additional disruptions. But under my terms protesters would have to shoulder a heavy burden of persuasion to escape censure.

Anti-apartheid activists justify their disruption on the ground that they have a legitimate interest in preventing South Africa from advancing its ideological war against racial justice. Appalled by the way the regime has stifled dissent at home, they contend that it would be unfair to allow the regime's officials to exploit the collegiate forum to praise a system that buries the hopes of nonwhite South Africans. Seared by the brutality of the South African state of emergency, they maintain that their actions constitute legitimate resistance to injustice and proper aid in the struggle for a more humane world.

Even if they are wrong on some levels, disruptive anti-apartheid activists are right in their empathetic response to those suffering under the horrors of the South African regime. And perhaps they are not wrong at all but correctly following wise intuitions that history will vindicate.

The writer is an assistant professor at Harvard Law School