The Supreme Court yesterday lightened the burden employers face in defending themselves against job discrimination suits when they hire or promote white males over women or minorities.

Employers do not have to prove that the person they hired or promoted was more qualified than the person passed over, the court said without dissent. To rebut an accusation of bias, they need only provide believable evidence that race or sex was not a factor in the decision.

Although the ruling is unfavorable to blacks, women and others bringing employment discrimination suits, civil rights lawyers said the decision merely differing interpretations among lower court judges. Justices Thurgood Marshall and William J. Brennan Jr., champions of stiff civil rights enforcement. joined in the 9-to-0 opinion.

The case stemmed from a sex discrimination complaint brought against the state of Texas by Joyce Burdine of Austin after she was passed over in favor of a man for a promotion to become project director of a public service job training program, and then was fired from a lesser position at the same time that a male employe was retained.

Burdine sued the Texas Department of Community Affairs under Title VII of the Civil Rights Act of 1964, which entails a three-step procedure. The person claiming bias must first show a likelihood of discrimination, a "prima facie" case. If successful, the employer must repond with an explanation showing that the employment decision was nondiscriminatory. The alleged victim then has a chance to disprove the explanation.

The dispute in the Burdine case and among lower courts around the country was over how convincing the employer's explanation must be.

The Fifth U.S. Circuit Court of Appeals in New Orleans adopted a tough standard for employers; it was this standard that was overturned yesterday.

Texas officials had responded to Burdine's charges by saying she was fired in part because of dissension with fellow employes and because of federal requirements that the program, which was federally funded, be made more efficient. The Fifth Circuit said the explanation failed to prove that the employer had not discriminated, in part because the explanation did not show that the male employes were better qualified than Burdine.

Justice Lewis F. Powell Jr., writing for the majority, said yesterday that this misinterpreted past Supreme Court rulings. The federal law prohibits discrimiation, he said, it "does not demand that an employer give preferential treatment to minorities or women. . . . It does not require the employer to restructure his employment practices to maximize the number of minorities and women hired."

Powell also said that the employer does not have to "persuade" or actually prove to a judge that an employment action was lawful. The employer "need only produce admissible evidence which would allow [the judge] rationally to conclude that the employment decision had not been motivated by discriminatory animus."

The ruling is not directly related to affirmative-action plans, which are either voluntary or imposed following a finding of past discrimination.

In another action yesterday, the court sidestepped the question of whether mental patients need special protections from discrimination. A group of Chicago mental patients, represented by a legal assistance organization, sought the special status, which would have prohibited denials of benefits to them without a compelling justification.

The case, Schweiker vs. Wilson, stemmed from a denial of $25-a-month Social Security allowances to mental patients in public institutions at the same time that patients in Medicaid-funded institutions receive the allowance.