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Goat-antimouse (DBA/2) SL2 lymphoma immunoglobulin was specifically cytotoxic to DBA/2-derived sl2 lymphoma cells. This anti-SL2 immunoglobulin, or a part of it, was cytophylic for peritoneal macrophages as shown by target cells adhering to macrophages incubated in vitro with the immunoglobulin. The target cells became free in the medium after incubation with 0.1% trypsin for 1 hour. In in vivo experiments, the incubation of immune or hyperimmune peritoneal cells with immunoglobulin from normal goat serum or from goat-anti-SL2 serum before use in immunotherapy decreased the number of DBA/2 mice surviving for more than 35 days an intraperitoneal injection with SL2 cells compared to the number of survivors inoculated with immune or hyperimmune cells only. These results show that, in immunotherapy with immune cells, we must consider the possibility that specific antitumor antibodies and antibodies not directed against the tumor cloud the therapeutic potnecy of the immune cells.