Bull HN Information Systems Inc., a unit of France's largest computer company, said yesterday it agreed to acquire Honeywell Federal Systems Inc., adding the McLean-based government contractor to the swelling ranks of U.S. companies being bought by foreign-owned firms.

Bull HN declined to disclose the terms of the agreement and said they are subject to final negotiation. An industry observer who asked not to be identified estimated the sale price at close to $100 million.

The U.S. government must approve the deal because Honeywell's 1,600-employee federal unit has a number of contracts with the Department of Defense. According to the federal unit's parent, Minneapolis-based Honeywell Inc., more than 70 percent of the subsidiary's 1989 sales of $274 million came from the Pentagon.

Thomas Gallagher, vice president and general counsel of Bull HN, said he does not anticipate any negative effect on Honeywell Federal employees. A Honeywell Federal statement said the sale won't affect its relationships with its customers or its strategy in the government market.

However, because federal law restricts the transfer of sensitive technology to foreign-owned companies, Bull HN said the Honeywell unit will be operated under a proxy arrangement in which its interests are represented by three independent shareholders. One of them is Earle Williams, president and chief executive of defense contractor BDM International Inc., a McLean-based unit of Ford Aerospace.

"Our job is to ensure that there's no transfer of inappropriate information from Honeywell Federal Systems to Bull HN," Williams said. "Since we're talking primarily classified information, the U.S. government decides what's release-able. All we do is make sure the information is protected from the foreign owners."

Gallagher said that the proxy holders also will be responsible for presenting business strategy and other ideas to the Honeywell unit, as well as reviewing its financial results.

The deal marks Bull HN's entrance into the hotly competitive federal contracting market and the end of Honeywell's active participation in the computer and computer services businesses.

Once a major player in the industry, Honeywell gradually has rid itself of all ties to that past.

The company in March 1987 sold a majority interest in its computer business to Compagnie des Machines Bull, Bull HN's parent, and Japan's NEC Corp.

"The federal systems unit did not fit our overall corporate strategy," said Dick Boyle, vice president of marketing and business development for Honeywell and chairman of Honeywell Federal Systems.

"We've made it clear that we're focusing on the {building systems} controls business.

"It is in the best interest of the customers and employees of our federal systems business and computer services business to be in an organization that wants to focus on information systems."

That's seems to be exactly what Bull HN plans.

"We feel it's very important in being a viable computer supplier in the U.S. to be a viable supplier to the government," Gallagher said. "That's a premier marketplace, and success there gives you recognition. And government requirements also keep our technical level at a high degree of advancement."

"This is a pure marketing move by Bull," said Marina Young, president of IDC Washington Inc., a Vienna-based technology and market research firm.