The antibody response to sheep erythrocytes was examined in normal mice and in mice with ascites tumor. Antigen stimulation from day 0 to day 7 after tumor transplantation was associated with a decreased immune response. From day 7 onward, no immune response occurred. PHA injection before antigen administration depressed the immune response, whereas PHA injection after antigen administration enhanced the immune response. Intraperitoneal administration of PHA caused a greater decrease in tumor volume. Further studies on the effect of PHA alone and in conjunction with other methods of therapy are being carried out at present.