UnitedHealth Group, the no. 2 American medical insurance company, said it will buy PacifiCare Health Systems for $8.14 billion, gaining the biggest manager of plans subsidized by the Medicare program.

UnitedHealth will exchange 1.1 of its shares plus $21.50 for each share of PacifiCare, the companies said yesterday. The deal is the second-biggest ever in health insurance, based on data compiled by Bloomberg.

The move will help Minnetonka, Minn.-based UnitedHealth boost profits with Medicare, the government medical program for the elderly and disabled, set to add a prescription-drug benefit January 1. Using private health plans to administer the program, Medicare may spend $60 billion next year on medicines for 30 million Americans, according to the government.

PacifiCare “has the goose that lays the golden eggs,” said Steven Sean Hill, who helps manage $3.5 billion at First Investors. “If you already have a lot of Medicare Advantage customers in one area, it’s a pretty safe assumption you are going to pick up a lot of prescription-drug plan users.”

The PacifiCare acquisition would triple the number of participants in UnitedHealth’s Medicare insurance plans to about 1 million, bolstering the combined company’s competitive position against rivals for the drug-benefit business, such as Aetna and Cigna.

UnitedHealth CEO William McGuire, 56, has made at least 10 acquisitions since 2004, based on data compiled by Bloomberg. Mr. McGuire, a physician who has led UnitedHealth since 1991, last year bought Oxford Health Plans for $5 billion and MidAtlantic Medical Services for $2.7 billion.

WellPoint was created by Anthem’s $20.8 billion purchase of WellPoint Health Networks last year, the biggest acquisition in industry history.

Cypress, Calif.-based PacifiCare’s stock jumped $4.41, or 6%, to $77.09 in New York Stock Exchange composite trading. PacifiCare shares had more than doubled in the 12 months before yesterday.

“The reason they have doubled is people are starting to understand the Medicare value to them,” said Les Funtleyder, a health care strategist at Miller Tabak & Company in New York. “It’s not going to be the wonderful program the Bush folks say, but over time this is going to work out for everybody, not just HMOs, but Medicare beneficiaries.”

UnitedHealth estimates the combined company would save between $75 million and $100 million in its first year because it will be able to trim administrative and technology-related costs.

Both companies have placed bids with the government seeking contracts for the Medicare prescription-drug program and will continue to promote those plans separately this year, UnitedHealth and PacifiCare said yesterday. PacifiCare’s plans for spending on promotions are unchanged, CEO Howard Phanstiel said.

Customers tend to be loyal to specific brands of Medicare plans, and UnitedHealth intends to keep at least some of PacifiCare’s plans intact after the acquisition, Mr. McGuire told investors.

PacifiCare has its own drug-benefits management company, which fills prescriptions for 5.6 million people, including 2.5 million who don’t belong to PacifiCare plans.

That may make it cheaper for United-Health to expand its prescription-drug business, and it may put pressure on Medco Health Solutions, the biggest American pharmacy-benefit manager. Franklin Lakes, N.J.-based Medco has a drug-benefits contract with United-Health, accounting for $6.5 billion, or 18%, of sales. Medco shares fell as much as 8.3% after the talks were reported.

Mr. Phanstiel, 56, last month proposed five Medicare drug-benefit insurance plans, including one with no monthly premium. Mr. Phanstiel said he expected 17 million Americans to sign up for the Medicare drug benefit next year, compared with a government estimate of as many as 30 million.

UnitedHealth has agreed to be the nationwide partner for the consumer group AARP, representing 36 million Americans ages 50 and older, for the Medicare drug plan. PacifiCare said in a 2004 filing the AARP plan would be its biggest competition.

“UnitedHealth is making a big bet on Medicare,” said Alec Vachon, who advises money managers on health policy as president of the Washington-based consulting firm Hamilton PPB. “Basically, overnight, UnitedHealth would triple its number of Medicare lives.”

Other companies with big Medicare roll books such as Humana also may become acquisition targets,