Shares of Envoy (ENVY) were trading at 32 on July 1, up from 22 two months earlier, a 45% jump. In part, the stock was rebounding from a bad fall--along with other health and technology issues. The rally was helped along by Envoy's mid-June takeover of Healthcare Data Interchange from Aetna U.S. Healthcare. But there's another reason pros are buying: a possible buyout.

''Acquisition of the Aetna unit makes Envoy the leading pure play in the fast-growing electronic processing of health-care claims,'' says analyst Joel Krasner of First Albany, a New York investment firm. Larger companies in the business, he says, will find Envoy an attractive target. His price goal for the stock this year is 40. In a buyout, it's worth 50 to 55, figures Krasner.

Envoy processes pharmacy insurance forms and third-party reimbursement claims for doctors and other health-care providers. It also provides eligibility verification and other services. To do that, Envoy uses proprietary software and microprocessor technology in dealing with 188,000 doctors, 44,000 pharmacies, 30,000 dentists, and 3,700 hospitals.

A New York money manager who has been buying shares in hopes of a takeover believes that HBO & Co., which supplies computer systems to health-care outfits, is sounding out Envoy. HBO, with a market capitalization of $6.7 billion, has been busy acquiring smaller companies in the same business. With Envoy's market cap at $519 million, ''it's an easy bite for HBO, whose sales this year are expected to exceed $1 billion,'' says this pro. Envoy is expected to produce revenues of $100 million this year.

With its recent acquisition, Envoy will become the clearinghouse for most of Aetna's claims. ''The acquisition will increase earnings, and we may raise our estimate for 1998,'' says Krasner.

He notes that, with 23.5 million patients enrolled with Aetna U.S Healthcare, each one filing one claim a year on average, ''the potential revenue opportunity is substantial.'' He figures Envoy will make 78 cents a share this year and $1 in 1998 (exclusive of nonrecurring items and goodwill associated with acquisitions), up from last year's 45 cents.