An experimental pill has shown remarkable promise against one form of leukemia in early tests, helping every patient in whom the standard treatment had failed. It could prove to be the first major treatment for the disease in 13 years.

The drug, called STI-571, is intended to treat chronic myelogenous leukemia (CML), which accounts for about 4,300 of the 30,000 new cases of leukemia in the United States each year, according to the Leukemia Society of America.

All 31 patients taking at least 300 milligrams a day have had normal white and red blood cell counts for as long as eight months, said Brian J. Druker of the Oregon Health Sciences University. Moreover, 30 of them had normal blood counts within a month of beginning treatment.

Most important, the medicine apparently eliminated cancer-causing cells in three patients, and reduced their number in six others. STI-571 could become the first major treatment for this kind of leukemia since interferon shots were introduced in 1986. Interferon, a natural protein that revs up the body's immune defenses, is the current standard treatment for CML and cures as many as one-third of those with the disease.

However, it can take two years of daily shots of interferon to bring about a remission. And interferon has many side effects, including pain and inflammation of the joints. The new drug's side effects are few and mild. And the drug is available in pill form.

Nobody knows yet if the new medication's effects will last or whether the cancer will develop a resistance to it. But researchers said it is an important development even if the effects turn out to be short-lived.

The reason: The medicine is designed to stop a certain enzyme that triggers the disease. Doctors said this approach breaks new ground by targeting the particular biological defect that causes the cancer.

Leukemia, when left untreated, produces vast numbers of immature white blood cells. These crowd out the mature white cells that protect the body from infection, as well as oxygen-carrying red blood cells and platelets, the tiny cells needed for blood to clot.

One specific enzyme, tyrosine kinase, is believed to cause more than 97 percent of all cases of CML.

Druker, who will present his findings in New Orleans on Sunday to the American Society of Hematology, said he has been working for a decade to find something that could stop that enzyme. He and researchers from Novartis Pharmaceuticals found such a compound six years ago and have been testing and refining it ever since.

The current tests in Oregon and at the University of California at Los Angeles and the M.D. Anderson Cancer Center in Houston were designed to find out whether the drug is safe and how much the body can tolerate. Patients took daily doses of 25 to 500 milligrams.

Fifty-four people, all of whom either never responded or were no longer responding to interferon, have taken the drug for at least four weeks. The white blood cell count dropped by more than half in every patient who has taken at least 140 milligrams of the drug a day, and all 31 taking at least 300 milligrams have normal white and red blood cell counts, Druker said.

The second round of human testing will start next month with about 200 patients, with 600 more to be enrolled later in the year.