Walk through the door of the plain, frame building in Riverdale and enter the world of supermarket law.

Unlike most law offices, the prices are posted on the wall for all to see: uncontested divorces, $150; simple wills, $35; court appearances, $125 - all far below the average for the area.

The office is the most recently opened branch of the Baltimore-based Legal Clinics of Cawley, Schmidt & Sharrow, the latest wrinkle in attempts to provide moderately priced legal services for the American middle class.

Started last August in Baltimore's Fells Point neighborhood, the clinics now have six branch offices, five in the Baltimore area and the one on Kenilworth Avenue in Riverdale. The partners plan to open a seventh clinic in Washington next month.

This new method of providing legal services received the seal of approval of the United States Supreme Court on Monday when it struck down bar association bans against lawyers advertising the availability and price routine legal services.

The Supreme Court case concerned a similar clinic in Phoenix, Ariz., and the Maryland Bar Association had filed a "friend of the court" brief opposing the Cawley, Schmidt & Sharrow clinics.

It is one of about 20 such legal clinics around the country. On the basis of the number of practicing attorneys associated with it (14, of whom six are partners) and branch offices, it is considered the largest. According to the founding partners, it was the first to open on the East Coast.

Its practice is aimed clearly at middle Americans, and its philosophy is firmly stated in the printed brochure available in the clinic waiting room: "In our society of high priced law firms and no-cost legal aid programs, only the rich and the poor have ready access to lawyers. People in the largest sector of our society - the middle income - must either do without lawyers and give up their rights or, when lawyers cannot be avoided, pay much more than they can afford."

Although this view may sound radical, it echoes an American Bar Association study quoted in Monday's Supreme Court decision, which said "the middle 70 per cent of our population is not being reached or served adequately by the legal profession."

William R. (Rick) Schmidt III, 28, one of the founding partners, said the Supreme Court decision will result in "a general lowering of fees. Because of the large number of lawyers there will be competition, and competition will result in more accountability and more accessibility.

The partners are preparing newspaper advertisements now. Until the Supreme Court decision, they had depended on brochures, newspaper and magazine articles and word of mouth to attract clients.

The Cawley, Schmidt and Sharrow clinics are geared to handled clients unaccustomed to dealing with lawyers. The first consultation is free - and fees are posted on the waiting room wall and listed in the brochure. The clinics are open evenings and weekends so clients don't have to take a day off work.

Moreover, they accept Master Charge and BankAmericard, and even allow payments on the installment plan - something more traditional law firms avoid.

"It's a winning idea for the middle class consumer," said Schmidt, who graduated from the University of Maryland Law School in 1975 with Linda Cawley, 28.

She said she first thought of the idea of providing low cost legal services when "I needed a divorce I couldn't afford." But the idea really became "cemented" during classes on legal ethics that discussed the large number of Americans who cannot get legal services.

They joined with Ronald Sharrow, 42, who already had a successful law practice, to set up the first clinic in Baltimore.

Since last August they have seen about 3,000 persons, and about half have signed on as clients. Most come in for divorces, for which they are charged $150 if it is uncontested and there are no children or property settlement. Children and property raise the fee for an uncontested divorce to $250.

Those fees are far below the average for Maryland, which a bar association study showed were $344 for the simplest kind of divorce. But some of the clinics' clients report that other lawyers quoted them fees of $500, $700 or even $1,200 for simple divorces. Cawley, who specializes in divorces, characterized the $1,200 fee as "a rip-off."

The lawyers also handle real estate settlements and are getting an increasing number of consumer law cases - people upset over defective merchandise of badly done home improvements.

For $25, lawyers will offer counseling on legal problems, such as traffic tickets and small claims cases in which it does not pay to have an attorney in court.

The Maryland concept is patterned after the oldest legal clinic in the country, run by Leonard D. Jacobs and Stephen Z. Meyers, both 30, which has four branches in the Los Angels area.