Fairfax County yesterday lost a last-ditch appeal to the U.S. Supreme Court and now faces a retrial of Justice Department charges that it discriminated against black and female job applicants.

The high court refused the county's request that it consider reversing a federal appeals court decision that U.S. District Judge Albert V. Bryan Jr. erred in finding Fairfax efforts to hire minorities and women adequate.

If the federal government is successful in its legal fight this time, Fairfax could be ordered to hire certain percentages of blacks and women in eight job categories, including officials and administrators, professionals and police and fire departments. Fairfax now has a system of voluntary goals.

Fairfax supervisors, who established an affirmative action program for the county under pressure in 1977, heard an analysis of the latest development from County Attorney David T. Stiff in a closed session at yesterday's regular board meeting.

"I was really disappointed," Supervisor Martha V. Pennino (D-Centreville) said afterward. "I thought we had adopted an affirmative action program and were making progress."

"I feel we have made excellent progress," Supervisor Joseph Alexander (D-Lee) said. "The first trial cost a lot of money, and so will this one."

Fairfax won the first trial, but that 1979 decision by Judge Bryan in Alexandria was overturned last year by a three-judge panel of the 4th Circuit Court of Appeals in Richmond. The panel, deciding that the county had used misleading statistics to defend its minority hiring record, ordered Bryan to start over again.

Yesterday's refusal by the Supreme Court to hear the county's appeal in effect upholds that decision.

At issue is the federal government's claim that discrimination could be proved by comparing the number of blacks and women who applied for jobs in Fairfax with the number actually hired. The figures used by Justice Department lawyers are estimates because records for previous years were destroyed by the county.

The appellate panel upheld that basis of comparison and rejected Fairfax's computations, saying they understated the black and female pool of workers and thus made the county's minority hiring look better than it actually was.

Fairfax officials have opposed the Justice Department suit bitterly, maintaining they already are making significant strides in bringing more blacks and women into higher-ranking jobs that as late as 1977 were generally monopolized by white males.

Personnel director Cornelius O'Kane acknowledged yesterday that there were "significant imbalances" in the county's work force until affirmative action was decreed in 1977. Since that time, he said, "I can't think of another jurisdiction that has made more progress."

O'Kane said, for example, that prior to affirmative action there were no black males, no black femalesw and a total of only five women in the category of officals and administrators as department heads and agency directors. Now, he said, of the 181 employes in that category, five are black males, three are black females and a total of 36 are females.

In the professional category, which includes land-use planners, social workers, economists and lawyers, there wre six black males, one black female and a total of 41 females prior to affirmative action, O'Kane said. The record now, he said, among the 1,893 professionals is 36 black males, 37 black females and a total of 1,005 females.

O'Kane said the county exceeded its hiring goals for blacks in five out of eight categories in the reporting year ending last June 30. It fell short in two categories, fire and police and service and maintenance. The county attained 81 and 85 percent of its goals respectively in those categories, he said.

The county attained only 50 percent of its goal in the hiring of black administrators, but he pointed out that the target was a small number, four.

When the appelate panel in Richmond threw out Judge Byran's 1979 decision, it did not comment directly on Fairfax's affirmative action hiring program.

Judge Byran, however, while acknowledging that some problems persisted, praised the program as "vigorous and successful" and refused to impose mandatory goals, as asked by the fedral government.