Injection of (CBA X A)F1 cells into neonatal CBA mice rendered them tolerant to skin grafts of (CBA X A)F1 origin. Limiting dilution analysis revealed a very low frequency of tolerogen-inducible cytotoxic T lymphocyte precursors (CTL-P) in spleens of tolerant mice. Two in vitro procedures allowed, however, the induction of tolerogen-specific CTL-P of high frequencies in tolerant mice: (a) the "by-pass" activation of spleen cells from tolerant mice by concanavalin A under short-term bulk culture conditions followed by culture of limiting numbers of activated responder cells, and (b) absorption of spleen cells from tolerant mice on monolayers of tolerogen-activated T cells from normal syngeneic mice. Furthermore, spleen cells from tolerant mice, recently challenged with a tolerogen-bearing skin graft, specifically suppressed the activation of tolerogen-reactive splenic CTL-P from normal CBA mice under limiting dilution conditions. These data confirm the presence of tolerogen-specific CTL-P of high frequency in tolerant mice and suggest their functional inactivation through a suppressive mechanism.