A study has been made of the ability of IgM and IgG to initiate ADCC by normal human peripheral blood lymphocytes (PBL) in a 51Cr release assay with sheep erythrocytes as target cells. The immunochemical purity of the IgM and IgG preparations was confirmed by ion-exchange and affinity chromatography. Passage of the IgM over DEAE-cellulose removed its C-dependent hemolytic as well as ADCC activity, whereas no activity was removed from IgM by passage over a Sepharose 4B-Protein A column. The opposite was observed with IgG; DEAE-cellulose removed no activity and the protein A column effectively depleted IgG activity in both C-dependent hemolysis and ADCC. The IgM-induced ADCC was compared to IgG-induced ADCC on the basis of kinetics, dose-response, and effector-target cell ratios required. In addition, it was shown that preincubation of lymphocytes facilitated IgM-induced ADCC but had little effect on IgG-induced ADCC. When preincubated PBL were used as effector cells the following was found: The time requirements for induction of cellular cytotoxicity by IgM and IgG were similar when the antibodies were used at concentrations near their hemolytic end point titers. Dose-response curves of IgM and IgG indicated that lymphocytes were induced to be cytotoxic more efficiently by IgM when the antibody classes were compared on the basis of molecules/target cell required to produce 50% lysis. The effector-target cell ratio producing optimal lysis in the assay was the same for both classes of antibody. In contrast, the cytotoxicity produced by freshly isolated PBL against IgM-sensitized target cells was significantly lower than against IgG-sensitized target cells in all comparisons.