Infection of mice with the murine sarcoma virus (Moloney) markedly suppressed the humoral antibody response to sheep erythrocyte antigen injected 10 days after infection, when tumor size was maximal, and on day 26, when primary tumors had partially regressed. Humoral antibody response was also inhibited when antigen was injected at the time secondary tumors and metastases were evident. No significant suppression of humoral antibody was seen when mice were injected with sheep erythrocyte antigen 5 days after virus infection. Inhibition of the cellular immune response of murine sarcoma virus (Moloney)-infected mice, as measured by the increased survival time of skin grafts, was also determined. Mice that were infected 5 days prior to grafting demonstrated prolonged survival of grafts, suggesting a suppression of cellular immunity. These mice had a graft survival time 14 days greater than noninfected controls. No significant prolongation of graft survival was seen in mice grafted at the times of maximum primary tumor growth, of primary tumor regression, or when secondary tumors had appeared.