ManTech International Corp., a Fairfax information technology company, plans to buy MSM Security Services Inc. of Greenbelt for about $5.3 million.

MSM, a 25-year-old company, conducts background investigations on applicants for security clearances. Its main clients include the National Security Agency, the U.S. Customs Service and the Defense Department's Defense Security Service. Last month, ManTech and MSM were awarded a joint contract with the DSS worth about $50 million.

"We see background investigations and security-related work as an important link in the value chain of security services," said Peter LaMontagne, a ManTech spokesman. "Because of counterintelligence concerns as well as homeland security requirements, it is important for the U.S. government to have a large population of cleared personnel, so we see this as an expanding market."

LaMontagne said it has not yet been determined whether layoffs will be necessary among MSM's 100 employees. ManTech said the firm recorded $19.9 million in revenue during 2002 and it expects that figure to grow 10 to 15 percent this year. MSM's founder and chief executive, Milton Mileski, will advise the firm through its integration but will not stay with ManTech.

William Earl Whaley, the president of MSM, will become president of the new unit, which is to be called ManTech MSM Security Services. The company will keep an office in Greenbelt.

Details of a provision that will allow MSM to receive additional payments based on its performance were not disclosed. The acquisition is expected to close in the next 15 days.

The news marks ManTech's fourth acquisition in a year and its second in as many weeks. Last week the firm said it agreed to buy Integrated Data Systems of Chantilly for about $57.7 million. That deal was completed Friday.

ManTech will report its fourth-quarter results after the markets close today. Shares of the company gained 16 cents yesterday to close at $15.73.