The serine/threonine kinase p70 S6 kinase (p70S6K) phosphorylates the 40 S ribosomal protein S6, modulating the translation of an mRNA subset that encodes ribosomal proteins and translation elongation factors. p70S6K is activated in response to mitogenic stimuli and is required for progression through the G1 phase of the cell cycle and for cell growth. Activation of p70S6K is regulated by phosphorylation of seven different residues distributed throughout the protein, a subset of which depends on the activity of p85/p110 phosphatidylinositol 3-kinase (PI3K); in fact, the phosphorylation status of Thr229 and Thr389 is intimately linked to PI3K activity. In the full-length enzyme, however, these sites are also acutely sensitive to the action of FKBP 12-rapamycin-associated protein (FRAP). The mechanism by which PI3K and FRAP cooperate to induce p70S6K activation remains unclear. Here we show that the p85 regulatory subunit of PI3K also controls p70S6K activation by mediating formation of a ternary complex with p70S6K and FRAP. The p85 C-terminal SH2 domain is responsible for p85 coupling to p70S6K and FRAP, because deletion of the C-terminal SH2 domain inhibits complex formation and impairs p70S6K activation by PI3K. Formation of this complex is not required for activation of a FRAP-independent form of p70S6K, however, underscoring the role of p85 in regulating FRAP-dependent p70S6K activation. These studies thus show that, in addition to the contribution of PI3K activity, the p85 regulatory subunit plays a critical role in p70S6K activation.