A partnership of Microsoft Corp. and four investment-banking firms last night all but inked a deal to buy a $500 million stake in Teligent Inc., a Vienna-based company that provides local telephone service to businesses, according to people familiar with the negotiations.

Under the terms being discussed last night, Microsoft would put forward $200 million and Hicks, Muse, Tate & Furst Inc., a private investment firm, would contribute $200 million. The other $100 million would be divided among three other private investment firms: Chance Capital Partners, DB Capital Partners and Olympus Partners. The three groups would claim 10 percent to 15 percent of Teligent, the sources said.

Negotiations going on in New York and by telephone were close to a deal late last night, but the sources cautioned that last-minute items could still derail an agreement. Microsoft and Teligent both declined to comment. "It's done," a source said. "They're just doing the paperwork."

The deal would hand Teligent, one of the fastest-growing alternative local service providers in the nation, a needed infusion of cash to continue building its network of wireless transmission facilities in markets across the country and overseas, the sources said.

The investment would give Microsoft another foothold in the lucrative world of telecommunications, as the software giant seeks ways to sustain its enviable run of revenue growth while gaining leverage in selling its products, an analyst said. The company has already made major investments in cable television, viewing them as conduits for high-speed Internet access.

"By having a say in some of the companies, they can basically make them use some of their technologies," said Rolf DeVegt, a telecommunications analyst with Renaissance Worldwide in San Francisco.

Hicks Muse would also expand its telecommunications holdings. Last year it took a $250 million stake in RCN Corp., one of the nation's largest Internet service providers.

Teligent, a Wall Street darling since going public two years ago, is headed by Alex J. Mandl, a former president and chief operating officer of AT&T. Under his direction, the company has focused on picking off small and medium-size business customers in major American markets. With rooftop antennas capable of rapidly moving huge flows of data--both voice calls and computer files--the company undercuts local telephone powers by bypassing their networks altogether. The company launched service a year ago and is now up and running in 34 markets, including Washington.

But erecting the equipment is costly. The sources said Teligent planned to use the cash to build out its network in domestic markets and to begin international expansion.