Less than a week after resigning from Hadron Inc., saying he wanted "to pursue other interests," former Hadron Inc. president L. Kenneth Johnson has rejoined rival Planning Research Corp. as senior vice president of program development in its government information systems division.

Johnson, 40, who left PRC in 1984 to go to Hadron, moved into his new office at the McLean professional services firm Monday. His resignation from Fairfax-based Hadron, also a professional services company, was announced a week ago Wednesday.

A Hadron spokeswoman said Johnson's resignation had been "amicable," and declined to say whether he had been negotiating with PRC before his resignation was announced. Hadron Chairman Dominic A. Laiti, who added the title of president after Johnson's resignation, could not be reached for comment.

In an interview yesterday, Johnson said he had handed in his resignation "several weeks" before it was officially announced by Hadron. He said he had told Laiti that he had been approached by PRC and wanted to negotiate with the company. Johnson would not comment on whether Laiti expressed disappointment, tried to convince him to stay or made a counteroffer.

Johnson said his time at Hadron, where he was president for 11 months, gave him valuable "management and profit and loss experience." But he said that as Hadron began to change its focus and market more toward commercial customers, he decided to leave. "I felt that that would obviously minimize the contributions that I might ultimately make to the corporation" as an expert in government contracting, he said.

He said he took PRC's offer because it provided him with "a significant challenge" and the opportunity to work again for Frank Calabrese, president of PRC's government information systems division and Johnson's boss before at PRC.

Johnson's PRC experience includes working as national sales manager for its realty systems division, as vice president of marketing with PRC's computer center, and as vice president of the company's government information systems division.

He left PRC to manage and operate Hadron's engineering and information systems division and was promoted to president of the company and chief operating officer in December.

While the two firms compete, Johnson said PRC's specialty is large-scale systems integration and software development, while Hadron's emphasis is on technical services, training and engineering assistance to government and commercial clients.

PRC, now owned by Emhart Corp., a diversified manufacturer, is 15 times the size of Hadron.

"We really only have a couple of common clients, and on those couple of clients we have in common {Hadron has} been providing support for them for some time and doing an excellent job," Johnson said.

"I don't think they are stealable, and I certainly don't want to" steal them, he said.

Executive cross-pollination between Washington professional service companies is not unusual, he said. "This is a transient business," he said.