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Phosphatidylinositol (PI) 3‐kinase has an 85 kDa subunit (p85 alpha) which mediates its association with activated protein tyrosine kinase receptors through SH2 domains, and an 110 kDa subunit (p110) which has intrinsic catalytic activity. Here p85 alpha and a related protein p85 beta are shown to form stable complexes with recombinant p110 in vivo and in vitro. Using a panel of glutathione S‐transferase (GST) fusion proteins of the inter‐SH2 region of p85, 104 amino acids were found to bind directly the p110 protein, while deletion mutants within this region further defined the binding site to a sequence of 35 amino acids. Transient expression of the mutant p85 alpha protein in mouse L cells showed it was unable to bind PI 3‐kinase activity in vivo. Mapping of the complementary site of interaction on the p110 protein defined 88 amino acids in the N‐terminal region of p110 which mediate the binding of this …