Marc Feldman sits behind a desk in a downtown law office looking just like any other Washington lawyer.

But Feldman, who is licensed to practice law in Maryland and Virgina, is not allowed to practice here. The D.C. Court of Appeals, which regulates the practice of law in Washington, refused to let him take the bar examination because he did not graduate from a law school approved by the American Bar Association.

Instead of going to law school, Feldman, 29, decided to follow the ancient and honorable, but fast disappearing method of apprenticing himself to an experienced lawyer - in this case a partner in a Charlottesville firm - since Virginia is one of a handful of states that allows reading for the law, as it's called, as an alternative to law school.

Reading for the law was the method followed by such luminaries as Thomas Jefferson and John Marshall, whose opinions as chief justice of the United States formed the basis of the American system of constitutional law. Abraham Lincoln, who also read for the law, called it "the cheapest, quickest and best way to become a lawyer."

Feldman is bout to challenge the D.C. court's refusal to allow him to take the bar exam in a federal court suit that could have national implications for the licensing of lawyers.

It will be a battle of legal titans, with two of Washington's biggest, best and most influential law firms involved. Covington & Burling has agreed to represent Feldman while Arnold & Porter will defend the judges. Both firms, who generally charge $100 an hour and up, are doing it pro bono - without any fee - because they feel the case is in the public interest.

Chief Judge Theodore R. Newman Jr. of the D.C. Court of Appeals said he expects the case to end up before the U.S. Supreme Court.

"This court must give him some opportunity to show that his unique training and experience have provided him with adequate qualifications to practice law," said Feldmen's lawyer, Robert M. Sussman of Covington & Burling, in a petition to Judge Newman.

"What bugged me so badly," added Felman, "is that when it was all said and done, there was no opportunity at all to prove myself. It wasn't as if I was given a chance and failed to pass muster."

Newman, though said the court must set standards, such as graduating from an ABA-accredited law school, for taking the bar exam "to prevent the committee (an Admissions) and the court from assuming the practically impossile task of making separate subjective evaluations of each applicants' training and education."

Indeed, the vast majority of states requires graduation from an ABA-accredited law school before allowing someone to take the bar exam.

Only eight states allow some form of reading for the law, and many of them require at least one year of formal law school training as well. Virginia is one of the few states that allows someone to become a lawyer just by reading for the law, but successfully completing its program generally conveys permission to take the bar exam in that state and no other.

Feldman was given special permission, because he was working in the Baltimore Legal Aid Bureau, take the Maryland bar exam, which he passed.

Feldman's case is important not only to himself but to graduates of two unaccredited law schools in the Washington area - Potomac School of Law and International School of Law. The D.C. appeals court gave graduates of International a special waiver last year to take the D.C. bar exam, but said it would not do it again.

Furthermore, a ruling by a high federal court could help thousands of graduates of unaccredited law schools across the country become practicing attorneys since in most states admission to the bar depends on graduation from an ABA approved school. There are 164 accredited law schools in the country.

While the image of most people who read for the law is tarnished by the notion that they either could not get in, or flunked out of law school, Feldman deliberately picked that route.

He had heard horror stories about law schools - especially how they failed to equip their graduates while he was in college. "Somewhere along the line," he said, "I heard about reading for the law."

After spending 1 1/2 years as a VISTA voulunteer, Feldman headed for Rhode Island to apprentice himself to a lawyer there. Ten days after he arrived, the state legislature abolished that option - forcing Feldman to look elsewhere.

He landed in Virginia, where John Lowe of Lowe & Gordon accepted him as a law leader. Feldman was bright and articulate enough to persuade the University of Virginia - which initially opposed the idea - to allow him to sit in on courses to give him theoretical knowledge he couldn't get by reading law or working in an office.

He had, for example, no need for courses in criminal law or legal procedures - he got plenty of actual experience in those fields - but needed to study about estates and wills because the law firm didn't do much of that kind of work.

Feldman started in the law firm doing the most simple kind of research. In the end, he said, "I did everything but appear in court."

He did well enough to get a job during his third year as a law clerk to Chief Judge William B. Bryant of the U.S. Court of Appeals for the District of Columbia - a plum that generally goes only to the best law students.

After finishing his three-year program of reading for the law, Feldman took the Virginia bar exam in Feburary 1976.

"I worked my butt off," he recalled. "I don't think I ever worked in such a concentrated way in my life.I was frigntened to death of the bar. Remember, I hadn't taken an exam in years, and I was under the pressure of thinking I had taken a second-rate way to become a lawyer."

His work paid off. Feldman passed the bar exam the first time he took it, something only about 25 percent of Virginia's law readers do. He later passed the Maryland bar, which he found more difficult than Virginia's exam.

Then he came to Washington, where Bernard Fensterwald Jr. offered him a job last year as an associated in his law firm. It is a small Washington firm, which Feldman said offers him the kind of varied practice he wanted.

As a Washington lawyer, though, Feldman is in a legal limbo since he cannot take the bar exam here. Technically he is not allowed to practice at all. Yet he is a member of the bar of the U.S. Court of Claims and the U.S. Court of Appeals for the District of Columbia and can argue cases in those courts. He can also appear on behalf of clients before federal agencies.

But, under D.C. Bar regulations, he is not supposed to see clients in his Washington office - he maintains a separate office in his home in Virginia - nor can be practice in either the local Superior Court or U.S. District Court here.

To avoid charges of unauthorized practice, Feldman said he makes it clear both on his stationery and in his conversations with clients that he is not a member of the D.C. Bar. Moreover, he has not appeared in courts here nor signed court documents, and he is careful not to seek clients.

By all accounts Feldman is considered the type of person who would make a good lawyer. Even Judge Newman, in a letter denying his request to take the bar exam, called him "a personable, impressive and outstanding individual," and said he "had an exceptional opportunity for training."

Yet there remains what Feldman calls "the dark cloud" of having taken law, which is keeping him from taking the bar exam here.

Ironically, if he waits five years, he can be admittedwithout taking the exam on the basis of having practiced law for that amount of time.