We have developed a monoclonal antibody (BAT) to Daudi B lymphoblastoid cell line membranes. The antibody was selected for its ability to stimulate lymphocyte proliferation. Splenocytes of BALB/c or C57BL mice given i.v. injections of 10 micrograms/mouse of BAT exhibited increased proliferation and cytotoxic activity. A single i.v. administration of BAT monoclonal antibody 2 weeks after B16 melanoma cell inoculation resulted in a striking antitumor effect as manifested by the elimination of lung metastases and prolonged survival of the treated mice. BAT monoclonal antibody was also effective in the regression of tumors in mice bearing 3LL (Lewis lung carcinoma) and MCA-105 (fibrosarcoma). Transfer of 10(7)-10(8) splenocytes from mice that had been given injections of BAT to B16- or 3LL-inoculated recipients led to a reduction of lung metastases. Splenocytes from B16-inoculated mice that were cured by BAT were more effective than those from mice treated with BAT alone against recipients bearing either B16 or 3LL tumors. The antitumor activity of BAT is related to its immunostimulatory properties.