Doctors are one step closer to a new method of cancer treatment that will set the body's defenses homing in to destroy cancer cells, researchers at Damon Biotech announced yesterday.

The scientists said they have developed the first practical method for making large quantities of chemicals called "human monoclonal antibodies," which are expected to be powerful new agents to fight cancer and other diseases.

Antibodies are the chemicals in the body's defense system that identify foreign substances, lock onto them and trigger the body's immune attack on foreign substances. The body makes more than a million antibodies, each different and each capable of recognizing and attacking only one substance--one type of virus, one type of bacteria, and so on.

When the body is trying to rid itself of an invader such as a cold germ, it makes many antibodies, and their actions are not always predictable. But several years ago two British researchers devised a method of making in the laboratory antibodies of a single type that would attack a single foreign substance.

These were called monoclonal antibodies after the method used to make them.

Scientists hope one day to make such single-target antibodies against cancer cells. The antibodies thus created would be able to enter the body, seek out and kill tumor cells and not harm any other cells in the body. The theory has been called the closest thing to the "magic-bullet" treatment against cancer that man is likely to devise soon.

But, until the past year, laboratories could not make human monoclonal antibodies, and in recent months human monoclonals could be made only in quantities too small to be useful for treatment of disease.

Nigel Webb, a biochemist and general manager of Damon Biotech, a biotechnology company in Needham, Mass., said yesterday that by other methods it could take more than 250 gallons of tissue culture, a great deal of time and a laborious process of purification to grow enough antibodies to make only four or five shots of antibodies for treatment.

But using the new method it is possible now to make 100 times as much, 50 times purer, and with considerably less difficulty.

The method uses tiny tenth-of-an-inch hollow spheres. The cells that manufacture the antibodies are placed inside the capsules. Nutrients can enter the micro-capsules through tiny pores, and waste products can leave the same way. But the antibody molecules are too large to leave the capsule and so collect inside them.

When the capsules are full they can be rinsed and broken open, releasing the human antibodies. The substances in the capsules are about 50 percent pure antibodies, compared with previous methods, which produced only 1 percent pure antibodies.

Preliminary tests done at Stanford University with anti-cancer antibodies from mice have proved very successful in a few patients, reversing advanced cancers dramatically. But the trials were limited and it will be some years before similar trials with human or mouse antibodies might reach large scale.

Since monoclonal antibodies are chemicals that attach themselves only to target substances, they are also expected to be useful in a number of other techniques. For example, to locate a suspected but unseen substance in the body, radioactively labeled antibodies that would seek out only that substance could be injected and followed on X-ray screens.