Mitogen-stimulated spleen cells from newborn mice do not synthesize mRNA for the 55-kDa interleukin-2 receptor (IL-2R). The kinetic of development after birth of ability to synthesize IL-2R correlated well with the functional immaturity of T cells, as was tested by responsiveness to T-cell mitogen concanavalin A (Con A). This functional immaturity of T cells was not due to the activity of neonatal suppressor cells (NSC) which inhibited immune responses induced by mitogens or antigens. The suppressor cells did not inhibit proliferation of spleen cells stimulated with IL-1 or IL-2, nor did they inhibited expression of genes for tumor necrosis factor (TNF)-beta, TNF-alpha, and IL-2R in stimulated cells from adult mice. The results thus show functional immaturity of T cells in newborn mice and selectivity of the immunosuppressive action of NSC, which allow for production and for functional activity of cytokines at a time when the specific immune system is not functional because of both immaturity and a selective activity of inhibitory cells.