Culture of murine lymphoid cells without added antigen results in the generation of cells which suppress a variety of in vitro immune responses, such as the mixed lymphocyte response (MLR) and the generation of alloreactive cytotoxic T cells (CTL). The ontogeny of this phenomenon was studied. Cells which suppressed the MLR after preculture were isolated from spleens and hematopoietic livers of fetal and young (less than 1 wk old) mice. On the other hand, the generation of alloreactive CTL could be suppressed only by precultured spleen cells taken from 1-w-old or older mice. The parallel between the development of the suppressor functions and the maturation of the responses they regulate, suggests a possible biological significance of the effect.