Monoclonal antibodies against a thymus cell differentiation antigen (Thy-1.1) were effective in the therapy of a transplanted mouse leukemia. Passive immunization resulted in high titers of cytotoxic antibody in the serum of treated mice and the suppression of metastatic tumor cells. The tumor-suppressive effects of the monoclonal antibodies were amplified by the administration of exogenous complement. This combined antibody and complement therapy resulted in the cure of leukemia in a significant proportion of the treated animals.