Medical scientists are on the verge of creating in the laboratory a new breed of antibodies that promise to become a major new tool in the early diagnosis and treatment of cancer.

Scientists say the new antibodies cover such a broad range they could be useful against such different diseases as arthritis and multiple sclerosis. They have already been used at the Stanford University Medical Center to detect fetal cells in a mother's blood as early as 15 weeks after conception, holding out the possibility of a simple, safe and inexpensive test for birth defects earlier than is now possible.

The Stanford researchers who have used the new antibodies to detect 15-week old fetal cells believe the antibodies' greatest promise lies in the quick detection and treatment of childhood leukemias, which are so numerous and varied that by the time they're identified the child's disease often is far along. The new antibodies may enable doctors to distinguish between leukemias almost from the onset of the disease.

"Right now, we can cure 50 percent of childhood leukemias but we can't cure the rest," Stanford's Dr. Leonard Herzenberg told science writers at a gathering sponsored by the National Science Foundation last week in Palo-Alto, just off the Stanford campus. "I think with this new technique we've got a real chance of beating the whole childhood leukemia thing."

Herzenberg said the new antibodies should become a major tool against such poorly understood bacterial infections as Legionnaires' disease.

"This was an unknown disease until a few years ago and we still don't know where these bacteria are hiding," Herzenberg said. "I think with this new method of creating antibodies we're going to be able to find them quite readily."

The new laboratory antibodies are unlike any created by the human body. They are called "monoclonal," meaning they are identical antibodies formed from the division of a single cell. All antibodies created in the human body are polyclonal, each one different because each has come from the division of a different cell.

The importance of monoclonal antibodies comes from the role of antibodies; they are used in most of the clinical tests performed today to detect human disease because they attach themselves to diseased cells and mark them. They are also used to type a person's blood before transfusion and a person's tissue before organ transplant. Without an antibody test, for example, kidney transplants would be next to impossible.

Because a human antibody is made up of a mix of many different cells, the tests are subject to a wide range of error. More important, diseased cells cannot be identified quickly because there might not be enough of them for the random-seeking polyclonal antibodies to find.

Not so with monoclonal antibodies, which seek out and tag cells in tailor-made fashion. Theoretically, a monoclonal antibody can be produced that will put a specific tag on most human diseases. A monoclonal antibody, for example, could be used to tag only acute lymphoblastic leukemia.

The remarkable specificity of these antibodies is expected to make them a valuable probe of the body's immune system and of cells in the body which are known to predispose people to a variety of immunologic diseases.

These diseases include ragweed hayfever, several kinds of arthritis, juvenile diabetes, multiple sclerosis, psoriasis, chronic hepatitis, and cancers such as most leukemias and Hodgkins disease, which is a cancer of the lymph glands.

One of the most dramatic applications of monoclonal antibodies is expected to be in the diagnosis of cancer. Dr. Irving Weissman of Stanford has already demonstrated that a mixture of monoclonal antibodies labeled with a radioactive tag can test for the presence of tumor cells in mice. His test can detect as few as one million leukemia cells, which is a thousand times more sensitive than any test now in use.

Stanford's Dr. Ronald Levy has used monoclonal antibodies to stain tumor cells in bone marrow taken from patients with cancer. Before staining, the cells were invisible in conventional diagnostic tests. The monoclonal tests tagged the cells almost immediately.

Monoclonal antibodies have revived hopes that a "magic bullet" can be developed to treat cancer. Doctors could bind drugs and toxic chemicals to antibody molecules and send them piggyback directly to the tumor. The tumor cells would then be killed with a minimum damage to surrounding tissues.

Herzenberg repeated that the best hope for monoclonal antibodies is in the rapid diagnosis of childhood leukemias, which are so different from each other and gallop so easily out of control even after seemingly being beaten by chemical treatment.

"You treat a childhood leukemia with drugs, you find the leukemia goes away and then months later reappears," Herzenberg said. "What you'd like to do is detect the few remaining cancer cells after that first treatment and with monoclconal antibodies we just might be able to do it."