Cultured human melanoma, lung carcinoma, and colon carcinoma cells were isotope labeled and incubated with a combination of effector cells and mouse monoclonal antibodies to tumor-associated cell surface antigens. The former were derived from the peritoneal cavity of mice or from peripheral blood of healthy human subjects. Monoclonal antibodies MG-21, 96.5, and L6, which are IgG3, IgG2a, and IgG2a, respectively, were all cytolytic when added in the presence of mouse effector cells to target cells expressing the relevant antigens. MG-21 and L6 were cytolytic also with human effector cells, while monoclonal antibody 96.5 was not. The effector cells attached to plastic surfaces, stained with neutral red, were peroxidase positive and mediated their effect over a 24- to 72-h time period as compared to the 4 h generally sufficient for antibody-dependent cellular cytotoxicity by natural killer cells. In tests on human effector cells with a fluorescence-activated cell sorter, they stained with antibody LCM-3C10 to the CD14 antigen, as well as with antimonocyte antibody 61D3. The cytolytic effect of human effector cells and antitumor antibody was not abolished by incubation with antibodies FC2 or 60.3 to CD16 and CD18, respectively, known to interfere with the antibody-dependent cellular cytotoxicity activity and natural killing of natural killer cells. This suggests, together with the other findings, that the effector cells were macrophages.