Terms call for Healtheon to pay $150 million in stock for privately held Actamed.
Healtheon, whose backers include venture capitalist Kleiner Perkins Caufield & Byers, has been gearing up to go public.
Last July, it bolstered its management team, which included the naming of Mike Long as CEO. Long previously was CEO of CSC-Continuum.
Last month, Actamed, which provides healthcare information networks, signed a pact with SmithKline Beecham for laboratory orders and results software. The company has 200 workers.
The buyout, which requires regulatory and stockholder approval, is expected to be completed by May. Clark remains chairman of the merged company. Mike Hoover, CEO of ActaMed, will become president, while Long will be CEO.