On rare occasion, a conjunction of legal and political developments offers certain fortunate citizens the opportunity to strike a major blow for the common good without taking a dime from their pockets. Today, April, 1, 1981, presents just such a chance. Doctors, lawyers and other professionals who live in the suburbs will begin receiving checks from the D.C. government -- the repayment of the city's nonresident professional tax, which was collected and subsequently declared illegal.

These checks will flow from a municipal Treasury that even Ronald Reagan would recognize as "truly needy." The checks, totaling about $40 million, will flow to people who generally fit the label "truly well off."

From these ironic circumstances arises the opportunity for the suburban professionals to make a bold, generous and public-minded gesture of good citizenship: Don't cash the checks.

There is no denying that, legally, these people have the right to cash their checks. They paid in good faith when the tax was instituted, and they won fair and square when it was challenged in court. The city might complain about the general unfairness of a Congress that permits Maryland and Virginia to tax commuters if they choose to, but denies D.C. the reciprocal right. But that's the way it is.

As a practical matter, however, the tax repayments can only be viewed as a windfall for the commuters. In cash-flow terms -- that is, out-of-pocket expenditures -- tearing up the checks won't cost them a thing. Moreover, it is probably safe to say that the financially sophisticated professionals who paid the tax wrote off every penny of it as a deduction on their income tax returns.

Leaving aside the legal and financial aspects, the principle that should govern here is fundamental fairness. The accountant from Fairfax who drives into the city to ply his trade at a prestigious downtown address is getting value at the city's expense. He expects and receives the protection of city police and fire fighters, drives on city streets, pollutes city air, listens to city-sponsored lunchtime concerts and runs straight to the city courts when a client fails to pay a fee. Except for the minute portion of his rent that goes to city taxes, he never pays for any of this.

To their credit, commuters do not try to deny that they derive tax-free benefits from the city. Instead, they brag about it. Fairfax County buys full-page ads in glossy magazines boasting of its proximity to Washington. The whole point is that you get the city without paying for it.

It is probably fruitless to suggest that the commuters should voluntarily assist the city every year. But in this particular instance, it should not be too much to ask a group of generally comfortable professionals to forego their windfall and help the District in an hour when the city is slighting its poorest victims for lack of funds.

No doubt the suburban lawyers who receive D.C. checks today will remind their city friends of Judge Learned Hand's famous declaration: "Anyone may so arrange his affairs that his taxes shall be as low as possible." But Judge Hand also observed that law-abiding society must rest on "the collective conscience of the community." He liked to talk about the importance of "that general spirit of fair play that unifies a nation under the law."

In the general spirit of fairness and community and regional unity, our commuting professionals could perform a simple but noble deed today. Tear up the check.