The U.S. General Accounting Office announced yesterday that it has agreed to pay a total of $3.5 million to about 300 present and former black employes who successfully charged the congressional watchdog agency with racial discrimination in promotions.

Believed to be the largest award in a discrimination case brought by black professional employes, according to an attorney for the plaintiffs, the settlement also requires GAO to promote 32 blacks to a higher job grade by October and to study and revamp, if necessary, its current promotion system.

The $3.5 million award, announced by Comptroller General Charles A. Bowsher, will be divided among black present and former employes depending on how long and at which grade levels they were eligible for promotions and whether they still work at the agency. Payments are expected to range from several thousand dollars to about $20,000.

The agreement resolves claims filed in two class action suits that accused GAO of denying its black employes equal opportunity for promotions to the agency's upper-level supervisory positions and challenged GAO's use of two competitive promotion systems.

The suits were filed by two GAO evaluators, Julian Fogle, who works in the agency's San Francisco office, and Tyrone Mason, who is employed in the agency's headquarters here. The focus of both suits, one of which resulted in a finding of discrimination by an Equal Employment Opportunity Commission hearing officer last October, was the system by which evaluators are promoted within the agency.

Evaluators, who comprise the bulk of GAO's professional staff, are the employes who analyze and evaluate government programs and perform audit functions.

Fogle's suit, which resulted in the discrimination finding by the EEOC officer, alleged that blacks had been unlawfully discriminated against since 1976 in seeking promotions to supervisory jobs. Mason's suit, which had yet to come before a hearing officer, challenged the promotion system adopted by GAO in 1983.

"If you look at the record from an educational point of view, blacks [at GAO] are very well qualified in the area of auditing," Mason said yesterday. "Yet they didn't progress."

Out of 3,577 evaluators at the agency in October, 1983, according to the most recent GAO figures, 1,993 held the higher paying, supervisory GS-13 to GS-15 grades. Of these, only 79 employes were black. In the lower-level, GS-7 to GS-9 grades, however, 65 out of 318 employes were black.

Kerry Scanlon, a lawyer at the Washington Lawyers' Committee for Civil Rights Under Law who helped represent GAO's black professionals, said blacks at the agency would generally progress, along with whites, from a starting GS-7 or GS-9 position until they reached the GS-12 level. Under federal civil service regulations, these were noncompetitive promotions that were automatically handed out each year on the employe's hiring anniversary.

But above GS-12, Scanlon said, promotions to the higher paying, supervisory jobs in the agency were based on competitive evaluations.

Promotions at this level, according to the EEOC hearing officer, were based on "largely uncontrolled subjective judgments" in which blacks were significantly disadvantaged.

In a similar class action suit settled in 1980, GAO agreed to pay $4.2 million to about 600 blacks who charged that they had been kept in low level GS-2 and GS-3 grades for years and never promoted into the agency's professional ranks. .