A new cancer treatment that activates the body's defenses, turning white blood cells into "killer cells" that attack tumors, has shown highly promising results in its first experimental test in patients, the National Cancer Institute announced yesterday.

The institute reported that use of the novel technique on 25 patients produced measurable reductions -- by more than 50 percent -- in the tumor sizes of 11 of them. The cancers were so advanced that they could not be treated with conventional drug or radiation therapy.

One patient with a severe form of skin cancer called melanoma showed complete disappearance of widespread cancer for at least 10 months, and nine showed partial responses in colon, kidney and melanoma cancers that had spread to the lungs or liver. A lung cancer patient was also helped.

"I don't know where this is going to lead," said Dr. Steven A. Rosenberg, the head of the research team and chief of surgery at NCI. "It's clearly a beginning and new direction," one of the first wholly new approaches to emerge since cancer drug development began three decades ago. Rosenberg gained public fame last summer as the medical spokesman for the team treating President Reagan for colon cancer.

Institute officials and outside experts hailed the initial signs of success as a major development in the long, often frustrating, struggle to develop new therapies that might make inroads in the war against the most common but difficult-to-treat deadly cancers.

But they cautioned that the findings are still preliminary. A limited number of patients have been studied for periods of 6 weeks to 10 months. The procedure is now very expensive and time-consuming, and may have major side effects.

It will take several years to determine how effective the new immune therapy will be in treating different cancers.

Last night, NCI spokesman Paul Van Nevel expressed concern about hundreds of telephone calls that are already flooding into the institute from cancer patients, emphasizing, "We're not able to take anyone else in the study at the moment."

Rosenberg said that he began laboratory tests with animals in 1978 using a protein known as interleukin-2, which is produced in small amounts in the body by certain immune system cells.

In humans, the laborious technique involves hooking a cancer patient to a machine that circulates the blood and removes only white blood cells. The procedure is painless but long, said Rosenberg, removing about 10 billion white blood cells over a four-hour period.

The white blood cells are cultured for several days in a solution containing a genetically engineered version of interleukin-2. The process apparently turns some of the cells into "killer cells" that selectively attack abnormal growths such as cancer.

The "killer cells" are injected back into the patient, along with interleukin-2, which apparently continues to stimulate their growth. The procedure must be repeated over a period of weeks.

Only tiny amounts of interleukin-2 are naturally available. Thus full-scale human research was hindered until a genetically engineered version made larger amounts available in 1984.

"It is the most interesting and exciting biological therapy we've seen so far," said NCI Director Dr. Vincent T. DeVita Jr. "It's working against very common malignancies and showing a very powerful effect."

Dr. Bruce A. Chabner, head of NCI's division of cancer treatment, said Rosenberg's work represents the most convincing demonstration in humans that an immune therapy can help turn the body's own natural defenses against tumors.

A slightly more cautious note came from Dr. Michael Colvin, professor of medicine at the Johns Hopkins Oncology Center, who noted that there had earlier been enthusiasm about other biological agents, such as interferon, that turned out to have more limited applications. But, he said in this case, the "initial batting average appears better."

Chabner said that the institute plans over the next year to begin human tests at several cancer centers around the country, in addition to Rosenberg's effort at the institute's clinical unit in Bethesda. New human studies also began this week at the government's Frederick Cancer Research Facility, but he said that at present they are not looking for additional volunteers.

The new findings are published in this week's issue of the New England Journal of Medicine in a "special report." Editor Arnold S. Relman said that it was only the second time that the journal had published preliminary evidence in this form.

"We think it's interesting and important enough for our readers to hear about it now, even though the final story has not been told. We'll have to wait awhile longer before we know whether this is going to be an effective treatment or not," he said. "It's an original approach . . . . Clearly something is happening that has not been seen before."

Rosenberg is one of many scientists who have been studying new biological substances that might be useful replacements or supplements to the conventional cancer therapies involving surgery, toxic anticancer drugs, or radiation.

Rosenberg said that during several weeks of therapy, patients experienced a variety of side effects, including major weight gain due to fluid retention in 16 of the 25 patients. In all patients, the adverse side effects disappeared when the treatment stopped, the NCI team reported.

The new immune therapy showed no effect at all in one patient with cancer of the esophagus and four patients with sarcomas, or soft tissue cancers often in the arms and legs.

It was also announced yesterday that Rosenberg will share the prestigious $100,000 cancer prize awarded annually by industrialist Armand Hammer, head of the President's cancer panel, to the scientists deemed to have made the greatest contribution toward a cure for cancer.