Human sera enhanced spontaneous cell-mediated cytotoxicity (SCMC), while anti-IgG (Fab') 2 treatment decreased this cytotoxic activity of human lymphocytes for an in vitro growing cell line (K--562). Trypsin treatment of the effector cells considerably decreased the cytotoxic potential. However, a significant cytotoxic activity could always be found in serum-free medium. While these findings suggest the involvement of antibodies in the SCMC, they also reflect the existence of serum-indpendent (sui generis) SCMC activity of lymphocytes. Removal of SCMC of Fc receptor bearing effector cells was performed by target cell adherence (rosetting). Separation of the target cell-bound lymphocytes was done by centrifugation on special Ficoll gradient. The depletion of SCMC effector cells resulted in a 62% reduction of SCMC and in a 39% reduction of ADCC. On the other hand, removal of Fc bearing effector cells showed a similar reduction in both ADCC (66%) and SCMC (78%). Our results suggest that SCMC represents a complex activity, arising partly from the interactions of certain serum-derived or lymphocytes surface-bound antibodies and partly from a spontaneous cytotoxic function of the effector cells. It is possible that the effector cells involved in both SCMC and ADCC derive from the same lymphocyte population and the differences are due mainly to the lower number of SCMC effector cells.