At about dawn on May 29, 1975, in Pittsburgh, a car carrying two black men was stopped by two white police officers for alleged traffic violations. A violent dispute, arrests and convictions followed.

Such episodes are commonplace around the country. But the one in Pittsburg has put before the Supreme Court a question it hasn't answered up to now:

If police officers misuse authority entrusted to them by a municipal government to deprive citizens of constitutional rights, can that government be held accountable, in the federal courts, under a Reconstruction-era civil rights law?

In the Pittsburgh case, the occupants of the car were brothers Glenn R. and Harvey L. Mahone. They allege that solely because they are black, Police Officers Davis S. Waddle and Albert B. Ellway arrested them without probable cause to do for and abused them with racial epithets and other verbal harrassment - and also with fists and nightsticks. Their lawyer said they were treated in a hospital.

Then, the brothers say in a brief in the Supreme Court, the officers threw them into a van, drove them to a station house and charged them with speeding and following too closely behind another car. The Mahones claim the charges were false. They were convicted the same day, on the basis of testimony by the officers that, they charge, also was false.

Later, claiming they had suffered bodily injury and mental anguish as a result of "a racially motivated pattern of police misconduct . . . against black citizen," the Mahones went into federal court to sue for damages.

They based their suit on various constitutional and legal grounds. But the 3rd U.S. Circuit Court of Appeals, accepting the allegations in the Mahones' complaint as true, focused on one issue: did the brothers have a claim against the city of Pittsburgh under what is known as section 1981 of the U.S. Code?

Section 1981 says blacks "have the same right in every state . . . to the full and equal benefit of all laws and proceedings for the security of persons as is enjoyed by white citizens, and shall be subject to like punishments, pains, penalties, taxes and exactions of every kind, and to no other."

Because of their race, the Mahones contended, they were subjected to official inflicted "punishment, pains, penalties" and denied the "equal benefit" of laws and proceedings guaranteed by the secion.

In modern times, neither the Supreme Court nor any federal appellate court has considered such a contention.

On Aug. 24, 3rd Circuit decided, 2 to 1, that the Mahones were right: ". . . the facts alleged fall within the broad language of both the 'equal benefits' and 'like punishment' clauses of section 1981."

In the opinion for the majority, Judge Max Rosenn worte that 1981 derived from the Civil Rights Act of 1866, which "was not intended to have merely limited effect; rather, it was to eradicate all discrimination against blacks and to secure for them full freedom and equality in civil rights."

He said that section 1981 makes the city liable because the Mahones' complaint was "based not on private acts of violence, but instead on official misconduct under color of state law by the city's police officers."

The city, in its petition for Supreme Court review, denied that it is liable, citing a grant of congressional immunity recognized by the court in a 1961 ruling.

Rosenn, however, said that the ruling was based on a different section of the U.S. Code, 1983, which derived not from the 1866 civil rights law, but from the Civil Rights Act of 1871.

The 1871 law had a narrower scope because it was enacted to implement only the Fourteenth Amendment, Rosenn said. The 1866 law, resting on the Thirteenth Amendment as well as the Fourteenth, was intended to abolish all of "the remaining badges and vestiges of slavery," he held.