THE Justice Department's conclusion that Section 504 of the Rehabilitation Act of 1973 does not prohibit discrimination based on actual or perceived capacity to transmit AIDS to others has been assailed as legally or medically flawed, mean-spirited, and antagonistic toward the gay community. The criticisms are misplaced. The Department's sober examination of the Rehabilitation Act reveals a commendable dedication to the rule of law and the belief that Congress, not bureaucrats, should be the foremost architect of national public policy.

Section 504 of the act denounces discrimination against ''otherwise qualified handicapped'' persons in programs or activities operated or funded by the Federal Government. A year after its enactment, Section 504 was amended to broaden the definition of handicap. Congress explicitly chose to protect from discrimination ''any person who has a physical or mental impairment which substantially limits one or more of such person's major life activities, has a record of such impairment, or is regarded as having such an impairment.''

In 1977, under President Jimmy Carter, the Department of Health, Education and Welfare issued regulations that confined the concept of handicap to physical or mental diseases and disorders, and excluded age, criminality, homosexuality or cultural or economic disadvantage. Moreover, the regulations explained, a physical or mental affliction is a protected handicap only if it ''substantially limits'' a major life activity, such as self-care, walking, seeing, hearing, speaking or working.

Neither the genesis of Section 504 nor its early administrative interpretation indicated an intent by Congress to consider whether persons who are actually capable - or simply believed to be capable - of transmitting contagious diseases should be protected against discrimination. Relying on the plain language and legislative history of Section 504, the Department of Justice reached the following irreproachable legal conclusions:

* That victims of AIDS, or acquired immune deficiency syndrome, are handicapped persons protected by the Rehabilitation Act because they suffer from physiological disorders that substantially limit major life activities.

* That victims of ARC, or AIDS-related complex, are likewise legally protected persons if the consequences of the impairment limit a major life activity.

* That persons who test positive for AIDS antibodies or others (including AIDS victims) are unprotected under section 504 from discriminatory treatment based on a genuine belief that they may communicate AIDS to others.

As the Department explained the latter conclusion, having a perceived capability of transmitting AIDS because of the presence in the bloodstream of AIDS antibodies or otherwise is not a protected handicap. Its reasoning was that such a believed capacity does not substantially limit an individual in performing major life activities. ACCORDINGLY, the Department declared, even if persons with AIDS antibodies suffer discrimination because of an erroneous belief that they will transmit the disease to others, Section 504 provides no remedy. That type of discrimination under the Rehabilitation Act, it noted, is akin to discrimination against persons who are left-handed or red-haired; it may be irrational, but Congress has not yet made it illegal.

Strident critics are complaining that the opinion will foster discrimination against homosexuals or others perceived to be potential carriers of AIDS. These complaints are misdirected for several reasons.

First, as the Justice Department stressed, state or local laws - or even other Federal laws - might proscribe discrimination against perceived AIDS carriers, even if Section 504 did not. Moreover, only Congress is empowered to amend Section 504 to include protection for people believed capable of transmitting AIDS - such policy decisions are constitutionally denied to the executive branch, under the separation of law-making and law-executing powers.

The gay community and other political minorities, as the foremost beneficiaries of the rule of law, should applaud, not condemn, the Department's opinion. By refusing to usurp policy making power from Congress, the Justice Department acted in the highest tradition of executive restraint.

Some critics of the opinion seem to be arguing that action by Congress to address statutory gaps or oversights is too slow, that Congress cannot anticipate problems such as AIDS, and that enlightened and efficient government thus requires an application of statutes beyond that intended by Congress.

Indeed, Congress does act slowly, often without foresight, and in ways detrimental to efficient government. But our entire consitutional scheme of checks and balances is intended to curb swift government action and to subordinate efficiency concerns to safeguard liberty and freedom. Waiting for Congress to address the AIDS question may tax the patience of many. But the department's interpretation of Section 504, unlike an executive ukase, has the redeeming virtue of legality.

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