A prominent Northern Virginia real estate agent was fined $2,000 yesterday and ordered to give up his brokerage license for 30 days on grounds he failed to disclose his personal interest in acquiring property near an Arlington Metro station.

The Virginia Real Estate Commission also found that James H. McMullin, 60, the former president of the Virginia Association of Realtors, had attempted to act for both the buyer and seller in the proposed transaction without letting either side know of his interest in the deal.

The commission, a semijudicial state agency, also cleared McMullin and one of his employes of charges that they engaged in "substantial misrepresentation" by their actions in the aborted 1977 transaction.

McMullin, a president of Real Estate Services Inc. of Arlington and an adviser to Republican politicians there, said through his lawyer yesterday that he will appeal the commission's findings to the Virginia courts. McMullin and Samuel G. Abramson, his employe, had denied all the charges.

The commission had the power to impose penalties ranging from a verbal reprimand to revocation of McMullin's brokerage license. The fine represents the maximum monetary penalty, according to commission director David Seitz.

The charges against McMullin chairman of Arlington's Economic Development Commission, stem from the aborted 1977 sale by Roger and Mildred Marquis of their home in Ballston.

That house is located next to a highrise apartment building built in 1964 and partially owned by McMullin. At a commission hearing in May, Marquis, who had bitterly fought McMullin's high rise, testified that he had vowed never to sell his home to McMullin because he believed the agent was "taking advantage of other property owners who didn't know the value of their property."

Marquis, a 68-year-old Justice Department official, said that because of ill health, he and his wife decided to sell their home and asked their nephew to handle the sale. Their nephew testified that he listed the house for $65,000 with a local brokerage firm.

According to commission documents, when McMullin learned the house was for sale, he called a wealthy Arlington businessman who agreed to make an offer on the house, ostensibly for a third party. Abramson was to act as a broker, according to the documents.

McMullin is accused of knowing that the house would be assigned to a dummy corporation of which he is sole stockholder. The commission found that he failed to disclose this to the Marquises, who testified that they thought the businessman -- not McMullin -- was the actual buyer.

The deal was never consummated because Marquis refused to sign the settlement contract because he believed he was being pressured to move too quickly. McMullin then sued the Marquises for breach of contract. That suit was dismissed in 1978.

During the commission hearing last May, Abramson testified that he did not know of McMullin's involvement in the dummy corporation. McMullin testified that he was only to act as a trustee until another business partner decided how to dispose of the property.