WHEN complexed with the inh acellular protein FKBP12, rapamycin is a potent immunosuppressant(1,2) and an inhibitor of a mitogen-stimulated signalling pathway that leads to activation of p70 S6 kinase(3-6) (p70(S6k)) and cyclin-dependent kinases(7-10) (CDKs). A recently cloned FKBP12-rapamycin-associated protein (FRAP/RAFT) is the likely mediator of these effects(11,12). Using FRAP variants that do not bind FKBP12-rapamycin, we demonstrate here that FRAP is a rapamycin-sensitive regulator of p70(S6k) in vivo and that the kinase activity of FRAP is required for this regulation. In addition, we show that FRAP autophosphorylates in vitro. Consistent with an essential role for FRAP kinase activity in vivo, autophosphorylation of FRAP is inhibited by FKBP12-rapamycin. Deletion studies indicate that the kinase activity of FRAP alone is not sufficient for control of p70(S6k) and that an amino-terminal domain in FRAP is also required.