Allogeneic lymphocyte cytotoxicity (ALC), i.e., rapid rejection of i.v. injected allogeneic lymphocytes in unprimed hosts, is an example of NK activity. Apparently anomalous rejection patterns, such as acceptance of F1 hybrid cells by parental hosts and rejection of parental cells by F1 hybrid hosts in many strain combinations, would fit the hypothesis that the effector cells in ALC recognize the absence of certain self-molecules (passwords) rather than the presence of nonself determinants. However, cold target inhibition studies showed that ALC displays allospecificity: when a mixture of radiolabeled AO and DA cells were injected i.v. into euthymic or athymic PVG rats, adding a surplus of cold DA cells reduced killing only of labeled DA cells and vice versa. Furthermore, semiallogeneic cold target cells were ineffective in inhibiting elimination of fully allogeneic cells, which supports the argument against a modification of the hypothesis that self-determinants inhibit a postbinding stage of lysis. Finally, (DA x AO)F1 cells injected into (DA x PVG)F1 hosts were rapidly rejected, despite the fact that donor and host shared expressed DA determinants. In sum, our results show that a hypothesis based on inhibition of killing by self-determinants can only be sustained with extensive modifications, and favor the alternative mechanism that the effector cells positively recognize the presence of allospecific determinants on the target cell surface.