We have previously shown that a subpopulation of nylon wool nonadherent, lymphoid cells in normal mice binds selectively to tumor cell targets that are sensitive to killing by natural killer (NK) cells. In the present report these target-binding cells (TBC) were enriched or depleted by velocity sedimentation and the isolation of single target-effector conjugates indicated that the majority of TBC killed their targets. Furthermore, the frequency of TBC correlated well (r = 0.86) with lysis in a population during kinetics experiments. TBC resembled small, resting lymphocytes with membrane specializations in the area of target cell contact as indicated by electron microscopy and cytochemical techniques. Target cell binding occurred before lysis in kinetics studies and regenerated in parallel with the lytic potential of a trypsinized population devoid of surface Ig or θ-bearing cells. Cell contact was prevented in the presence of EDTA whereas metabolic inhibitors or inhibitors of serine proteases suppressed lysis with no effect on the frequency of TBC. Conversely, an interferon-inducing agent elevated NK-mediated cytolysis with no effect on the level of TBC. These results suggest that i) the majority of TBC that we detect represents the NK cell and ii) lysis and binding are independently controlled events. A tentative model of NK-mediated lysis is proposed.