The possible involvement of natural killer cytotoxic factors (NKCF) in the lytic mechanism of natural killer (NK) cell-mediated cytotoxicity (CMC) was investigated by studying the mechanism of NK resistance of variant clones of the YAC-1 cell line. The NK-resistant YAC-1 (YAC-R) clones were generated by prolonged culture of the initially NK-sensitive YAC-1 cell line in the presence of NKCF. The YAC-R clones were resistant to lysis by NKCF as well as lysis by NK cells in a CMC assay. The defect was specific for NK CMC because the YAC-R clones could still be lysed by alloimmune cytotoxic T lymphocytes. Experiments to determine the mechanism of NK resistance of the YAC-R clones indicated that they still possessed the NK recognition structures because they formed a normal number of conjugates with murine spleen cells. In addition, the YAC-R clones, like the parental YAC-1 cell line, were able to stimulate the release of NKCF during co-culture with spleen cells. However, the YAC-R clones, in contrast to YAC-1 cells, were unable to adsorb NKCF from cell-free supernatants of such cultures. Thus, it appears that the YAC-R clones are NK resistant due to a deficiency of NKCF binding sites. The present studies demonstrate that a NK-sensitive target cell must not only be recognized by the NK cell and stimulate release of NKCF but it must also bind NKCF for cell lysis to ultimately result. These findings support our model for the mechanism of NK CMC in which it is proposed that target cell lysis is mediated by NKCF released from the effector cell.