Antibody-dependent cellular cytotoxicity (ADCC) increased with the development of tumors in C3H/He mice bearing spontaneous breast cancer or the syngeneic hepatoma MH-134 and in C57BL/6 mice bearing the syngeneic Lewis lung carcinoma 3LL. This cytotoxicity decreased after treatment with guinea pig, monoclonal IgM anti-Thy 1.2 serum and complement to the non-cancer level thus indicating that the increased ADCC in mice with cancer seems mainly attributable to cells with the Thy 1 antigen. On the other hand, NK activity decreased greatly when mice had tumors. Treatment with monoclonal IgM anti-Thy 1.2 serum and complement showed no significant influence on the natural killer (NK) activity of spleen cells of mice bearing MH-134 cancer, but in the 3LL-bearing mice the activity decreased significantly.