Cigna Healthplan Inc., the nation's largest investor-owned chain of health maintenance organizations, said it has agreed to acquire a Maryland HMO.

Cigna Healthplan Inc., a subsidiary of Philadelphia-based Cigna Corp., would not disclose the terms of the agreement with Maryland Individual Practice Association Inc.

"We are involved in a large expansion plan," said Barbara Zbell, spokeswoman of Cigna Healthplan Inc. "We've either acquired other companies or started up our own HMOs in 15 states now and we want to be in 30 states by the end of next year."

Rockville-based MD-IPA, which serves 60,000 patients in the Washington-Baltimore area, is a so-called "individual" health maintenance organization. The company, which began operating as a Montgomery County HMO in 1981, has expanded to serve the Washington-Baltimore corridor.

"There would be marketing advantages in an agreement with a national HMO network," said Andrea Stillman, spokeswoman of MD-IPA. "We would be able to offer our members more choices of alternative health-care delivery because Cigna offers different kinds of health-care packages."

An "individual" HMO such as MD-IPA provides medical services to enrollees by doctors in private practice who are under contract with the association but who work out of their own offices rather than in health-care clinics. Other HMOs either hire their own physicians or contract with a large group of physicians that work exclusively for that organization, Stillman said.

Last year, MD-IPA's profit climbed to $250,000 on revenue of $19.5 million. Cigna Healthplan Inc. is a $600 million company that serves about 850,000 patients.