The roles of the alpha- and beta-isoforms of phosphatidylinositol (PI) 3'-kinase p85 regulatory subunit were studied with isoform-specific antisera in three model systems in which the insulin receptor mediates rapid phosphorylation of insulin receptor substrate-1 (IRS-1). Insulin receptor signaling stimulated the association of IRS-1 with p85 alpha protein, and p85 alpha-associated PI 3-kinase activity in 3T3-L1 adipocytes, and in transfected Chinese hamster ovary cells (CHO-T) and COS-1 cells expressing high levels of human insulin receptors. While not detectable in 3T3-L1 adipocytes, the p85 beta isoform was also found to associate with IRS-1 in response to insulin receptor activation in COS-1 and CHO-T cells. However, selective immunoprecipitation of p85 beta from unstimulated COS-1 or CHO-T cell lysates was accompanied by higher levels of PI 3-kinase activity than that associated with p85 alpha. Remarkably, the large stimulation of PI 3-kinase activity associated with p85 alpha (7.8 +/- 2.0-fold, n = 6) in insulin-treated CHO-T cells was not observed in p85 beta immunoprecipitates (1.8 +/- 0.6-fold, n = 6), and in COS-1 cells p85 beta-associated PI 3-kinase activity was completely insensitive to stimulation by the insulin receptor. These data suggest the novel hypothesis that binding of p85 beta to IRS-1 complexes in COS-1 and CHO-T cells does not mediate marked activation of PI 3-kinase activity as does p85 alpha.