McLean-based Primark Corp., continuing to shuffle its portfolio of businesses and cash since spinning off its huge Michigan Consolidated Gas division two years ago, yesterday sold a business that accounted for almost half its revenue last year to American Capital and Research Corp. of Fairfax.

Primark also has received antitrust clearance from the Federal Trade Commission to buy a potentially significant stake in forest products giant Champion International Corp., based in Stamford, Conn.

It was not clear whether the two developments were directly related, but Primark, which shrank from $2 billion in annual revenue in 1985 to $39.3 million last year after selling Michigan Consolidated Gas, has been openly looking to acquire new businesses with the more than $100 million remaining from the sale of the utility.

Primark will gain at least $11.6 million more from the sale yesterday of its 65-employee Health Information International (HII) division to American Capital. The division, which provides televised training programs to more than 1,000 hospitals, had revenue of more than $16 million last year, nearly half Primark's total.

Primark officials said they are interested in Champion only as an investment, at least for now, although Chairman Joseph E. Kasputys indicated to USA Today earlier this month that the company might expand its interest at a later date.

A Champion spokesman said that, as far as he knew, Primark had not yet bought any stock in the company. Primark officials declined to discuss their Champion holdings, if any. Champion is much larger than Primark, with revenue last year of $5.2 billion.

The FTC said it had approved an application by Primark, under the Hart-Scott-Rodino antitrust law amendments, to purchase at least $15 million, or 15 percent, of Champion's shares.

Investors are required to ask the FTC to review such purchases on antitrust grounds and must wait until the review is completed to purchase stock. As is not uncommon in such situations, the FTC granted Primark an early termination of the review period.

In trading on the New York Stock Exchange, Champion stock closed at $25.87 1/2 yesterday, down 37 1/2 cents, and with about 92 million shares outstanding, a 15 percent stake in the company would cost more than $350 million.

Primark stock was unchanged yesterday at $7.37 1/2, and American Capital fell 25 cents, to $10.25.

Primark's remaining businesses are an eclectic lot, ranging from financial services to gas storage facility leasing. In a statement, Kasputys said, "The sale of HII constitutes another step towards divesting businesses that do not meet our long-term strategic objectives ..." Company officials did not elaborate on its strategy.

Primark officials also would not elaborate on their interest in Champion, which makes paper and wood products and building supplies. But the interest seems a marked change from the company's last reported target: McLean-based government services provider Planning Research Corp., for which Primark briefly made overtures last year.

Primark is selling Health Information International to American Capital for a package of cash and American Capital stock. Primark will be restricted from selling the American Capital stock for a year, and then will be guaranteed a minimum selling price of $14 a share by the company.

Although the two companies declined to discuss the terms of the deal, Primark said the sale of the division will bring in at least $11.6 million in cash, tax benefits and potential proceeds from the sale of the American Capital stock.

Health Information International joins a growing group of health services businesses owned by American Capital, which also provides engineering and other professional services to government and commercial clients.