THE macrolide rapamycin induces cell cycle Gl arrest in yeast and in mammalian cells^1–3, which suggests that an evolutionarily conserved, rapamycin-sensitive pathway may regulate entry into S phase. In mammals, rapamycin inhibits interleukin-2 receptor-induced S phase entry and subsequent T-cell proliferation^4–6, resulting in immunosuppression. Here we show that interleukin-2 selectively stimulates the phosphor) lation and activation of p70 S6 kinase but not the erk-encoded MAP kinases and rsk-encoded S6 kinases^7,8. Rapamycin completely and rapidly inhibits interleukin-2-induced phosphorylation and activation of p70 S6 kinase at concentrations comparable to those blocking S phase entry of T cells (0.05–0.2 nM). The structurally related macrolide FK506 competitively antagonizes the actions of rapamycin, indicating that these effects are mediated by FKBP, which binds the transition-state mimic structure common to both rapamycin and FK506 (refs 4, 6, 9–11). The selective blockade of the p70 S6 kinase activation cascade by the rapamycin–FKBP complex implicates this signalling pathway in the regulation of T cell entry into S phase.