The lysis of human peripheral blood lymphocytes by rabbit antisera directed against the Fc Portion of human IgG has been demonstrated. A necessary condition for this lysis is an incubation of the cells at 37 C prior to the addition of antiserum. Rabbit antihuman IgG causes similar lysis to anti-Fe (äFc) but absorption studies with Fc and Fab indicate the cytotoxicity arises from the antibody molecules directed against the Fc portion of human IgG.