SummaryWe produced antibodies by injecting cancer cells obtained from Walker 256 tumor into rabbits; after several injections of the cell antigen, the rabbits were exsanguinated, and the serum fractionated. Antibodies to normal tissue were adsorbed out by exposure to normal tissue. Injection of the globulin fraction prevented “takes” of Walker 256 cells in slightly over half of rats inoculated with 10,000 cells. The globulin antibody solution produced a decrease in size by about one quarter when given to rats with tumors 2 to 3 cm in diameter, but the tumor masses did not disappear. The method was tried in 2 patients with decrease in size of the tumor but not disappearance.