This report describes the primary in vitro generation of human CTL that lyse TNP-derivatized autologous cells. Although in the majority of these studies, a direct cytotoxic response to the TNP-modified autologous stimulators was not achieved, in all experiments the addition of either allogeneic cells or soluble antigen triggered the generation of killer cells which destroy TNP-modified, but not unaltered, autologous targets. Fractionation of responder lymphocyte populations demonstrated that the cytotoxic activity was mediated by T cells. Killer cell specificity was tested by assaying for cytotoxicity to a variety of targets, and by blocking the cytolysis of TNP-altered autologous targets with various populations of nonradiolabeled cells. Results indicated that these CTL were cytotoxic for TNP-modified autologous cells but not unaltered autologous or TNP-modified allogeneic targets. The capacity of soluble antigen and alloantigens to facilitate the in vitro generation of altered-self reactive human CTL is not an isolated phenomenon. This "helper" effect has now been observed for the cytotoxic response to chemically modified autologous cells and MHC identical human leukemic blasts. It is possible that in vivo, similar responses to nonspecific antigenic stimuli may play a role in the maintenance of immune surveillance.