Tumor induction and immunity to tumors were studied following the injection of Moloney sarcoma virus (MSV) into mice whose B-lymphocyte functions had been suppressed by the chronic administration of anti-IgM antibodies. Two preparations of MSV were used; one which gives rise to tumors which uniformly regress in normal adult mice, and another which elicits progressively growing tumors in the majority of recipients. The tumor incidence, mean tumor size, and tempo of regression were not modified by treatment with anti-IgM. However, whereas tumors induced by the regressor virus were all rejected in 19 NRG-treated and 29 untreated recipients, continued growth was obtained in 2 of 23 B-lymphocytedeprived mice. Furthermore, in 9 additional mice from this group, apparent rejection was followed by tumor recurrence at the site of the initial tumor. Continued growth was accompanied by widespread metastasis. These tumors were freely transplantable to normal syngeneic recipients. Metastasis and transplantability were also detected in 7 of 24 anti-IgM-treated mice given progressor virus, but were not seen in the control animals. Recurrence and metastasis was obtained despite the presence of high levels of specific cytotoxic T lymphocytes in the spleen. It is concluded that B lymphocytes or their products play an essential role in host protection against MSV-induced tumors.