When a cognate peptide is added to a culture of the corresponding clone of CD8+ cytotoxic T lymphocytes (CTLs) having the appropriate major histocompatibility complex (MHC) each cell can serve as both an antigen-presenting target cell and as a responding CTL. Under these circumstances many of the cells die. The extent of cell death is greatly diminished by Ca2+ chelation (by Mg2 EGTA) and by mAbs to CD8 and to LFA-1. Cell death is also blocked by cyclosporin A and FK-506, but not by inhibitors of protein synthesis and gene transcription (cyclohexamide and actinomycin D respectively). In contrast to the stimulatory effect on cytolytic activity, proliferation of recently stimulated CTLs is profoundly inhibited by the cognate peptide, as well as by conventional target cells that are specifically recognized and lysed by the CTLs. The inhibition of proliferation is transient and is followed by enhanced DNA synthesis of surviving CTLs. Cognate peptides also elicit fragmentation of CTL DNA, and this effect is likewise decreased by cyclosporin A and FK-506. Thus the addition of a target, either in the form of a synthetic cognate peptide that associates with MHC proteins on intact CTLs or other cells, or in the form of a conventional target cell, can simultaneously stimulate cytolytic activity and inhibit the proliferative activity of mature CTLs.