A federal judge ruled yesterday that Prince George's County did not discriminate against a black who failed to win a job and one who was downgraded in the early 1970s.

U.S. District Court Judge Alexander Harvey II said the plaintiffs did not prove any of their claims of racial discrimination and that county officials had worked hard to correct any imbalances.

"Plaintiffs here have not proved that racial discrimination was the county's 'standard operating procedure--the regular rather than the unusual practice,' " Harvey wrote in an opinion rejecting the claims of the individuals as well hundreds of black employes and applicants they claimed to represent.

"Quite clearly," Harvey added, "blacks were underrepresented as county employes in 1972, but during the last eight years the county has made a diligent effort to catch up under very trying circumstances."

Prince George's County Attorney Robert Ostrom praised the decision as a "total vindication" of the county's affirmative action policy. Ostrom said a decision against the county could have cost between $1 and $15 million in damages, including back pay and attorneys fees.

The class action suit, which was heard for three weeks last October in Baltimore, was filed in 1977 on behalf of Patricia B. Allen of Upper Marlboro and Sylvester Vaughns Jr. of Palmer Park.

Allen charged that because of her race she did not get a secretarial job she had applied for and qualified for. She said she represented hundreds of "minimally qualified" black employes who were denied county jobs between 1972, when the law under which she sued became applicable to the county, and 1980.

Vaughns, a past president of the county's NAACP, joined the Allen suit in 1975 when he was downgraded during a reorganization of his department. He charged that a white employe in his department was transferred at the same time without any loss of pay. He contended he represented the interests of most of the county's 883 black employes.

Allen and Vaughns sought to prove that blacks are underrepresented in the county's labor force, given their availability in the area labor pool. They said the county's personnel practices discriminate against the hiring and promotion of blacks. And when blacks do win jobs, they argued, they are paid as much as $1,500 less than their white counterparts. The county argued that blacks are hired in proportion to the number of applications submitted by blacks.

In his 43-page opinion, Harvey reviewed the county's affirmative action plans dating to 1973 and said that although they had not been uniformly successful, "their lack of success must be attributed to factors beyond the control of county officials. On the one hand, there has been a dramatic increase in the number of black residents in the county. On the other hand, county officials have been met by demands by its citizens for budgetary constraints."

Harvey also questioned Allen's desire to work for the county, suggesting she was overqualified for the position she sought.

Allen, who is now a GS-11 administrative assistant for the Federal Communications Commission making three times what she would have as a county secretary in 1972, said, "first they don't hire blacks because they say you're not qualified. Then he said my motives were suspect . . . . You're either overqualified or underqualified. What do you have to do to work?"

Jeffrey Burt, chief counsel for Allen and Vaughns, said he would consider appealing the decision.