The Supreme Court ruled 7 to 2 yesterday that the Constitution does not require a state to let prisoners solicit memberships in a prisoners' "union," let the union hold meetings or let bulk mailings about the organization be made from the outside.

"Prison life, and relations between the inmates themselves and between the inmates and prison officials or staff, contain the ever-present potential for violent confrontation and conflagration," Justice William H. Rehnquist wrote in the opinion for the court.

"Responsible prison officials must be permitted to take reasonable steps to forestall such a threat, and they must be permitted to act before the time when they can compile a dossier in the eve of a riot," Rehnquist continued. "The case of a prisoners' union, where the focus is on the presentation of grievances to, and encouragement of adversary relations with, institution officials surely would rank high on anyone's list of potential trouble spots."

The decision came in a case involving the North Carolina Prisoners' Labor Union, Inc., which was chartered in 1974. It seeks to promote "charitable labor union purposes," to form a unit "at every prison and jail" in the state, to be a vehicle for the presentation and resolution of inmate grievances and "to seek through collective bargaining . . . to improve . . . working . . . conditions."

State law forbids the organization from bargaining over pay, hours of work and other conditions of incarceration. Prisoners become members simply by saying they are. They incur no dues or obligations. By early 1975, 2,000 of them at 40 different units had joined. Similar organizations have been formed in several other states.

The state did not resist membership or belief in the union, but proposed regulations prohibiting inmates from soliciting other inmates, union meetings and bulk mailings.

The union then sued in federal court, claiming that the rules would deny the free speech and assembly or association guaranteed by the First Amendment. It also alleged that the rules would violate the 14th Amendment's guarantee of equal protection of the laws, because the state allowed groups such as the Jaycees and Alcoholics Anonymous to meet inside prisons.

State officials testified that even if the union had no goals other than the ones it professed, inmate organizers [WORD ILLEGIBLE] become powerful figures who would be "in a position to misuse their influence" creating a "potential for a dangerous situation" that authorities could not control.

A three-judge panel, while acknowledging that state officials expressed their apprehensions sincerely, ruled for the union.

But Rehnquist emphasized that confinement and the needs of a penal instituation "impose limits on constitutional rights, including those derived from the First Amendment, which are implicit in incarceration." As for mail, he said, only bulk mailings, which are less expensive and more convenient than first-class mail, were being cut off. And, he emphasized, nothing required officials to treat inmate meetings of the union and, say, Alcoholics Anonymous, identically.

Justice Thurgood Marshall, joined by Justice William J. Brennan Jr., Dissented.