We have characterized the effector cells present in fresh adherent cell preparations from human peripheral blood that are responsible for the lysis of K562 and IgG antibody-sensitized P815Y target tumor cells. Purified lymphocytes and monocytes, separated from adherent cell preparations by using techniques that utilized light scatter properties and reactivity of the cells with monoclonal antibodies, were used as effectors for the spontaneous and antibody-dependent cytotoxicity. The role of monocytes was also studied at the single cell level. Our results indicate that when a subpopulation of adherent cells that is B73.1+ (NK cells), B44.1- (non-monocyte), with light scatter characteristics of lymphocytes, is removed from the total adherent cell population, the resultant purified monocytes are unable to spontaneously kill K562 target cells. This data suggests that fresh unstimulated adherent cell cytotoxicity against K562 cells is mediated by lymphocyte contamination in these preparations and that the cell responsible for this killing is probably the NK cell. In contrast antibody-dependent cellular cytotoxicity against P815Y target cells is mediated by monocytes and lymphocytes.