Injections of a powerful solvent and two other new treatments should make gallbladder surgery unnecessary for many of the 500,000 Americans who undergo the operation each year, researchers reported Thursday.

Mayo Clinic doctors described in the New England Journal of Medicine their use of methyl tert-butyl ether, a substance that until now has been used largely as a gasoline octane booster and as an industrial solvent.Injecting it directly into the gallbladder, doctors found that it quickly dissolved large stones, freeing patients from their painful condition.

"We have learned that we can use the technology successfully in a broad range of anatomical circumstances," said Dr. Johnson L. Thistle, who pioneered the procedure. "We have become quite comfortable with the feeling that we can do it safely and effectively."

However, the experimental technique has competition from two other therapies that also promise to replace gallbladder removal, which until now has been the standard treatment for gallstones and trails only Caesarean sections as the most common form of abdominal surgery.

A stone-crushing machine called a lithotriptor, already widely used for kidney stones, is being tested against gallstones, and a stone-dissolving drug was recently approved for routine use.

"All three of them need to be considered," said Dr. Gerald Salen of the Veterans Administration Medical Center in East Orange, N.J. "They offer an alternative to an operation. Most patients can have their stones treated with one of these techniques."

Salen said the three treatments may be useful for many of the 350,000 or so people who undergo elective gallbladder surgery annually. In 1987, 526,000 Americans had their gallbladders taken out. Surgery will still be necessary for 150,000 people who need emergency treatment.