Insulin receptor substrate (IRS) proteins are tyrosine phosphorylated and mediate multiple signals during activation of the receptors for insulin, insulin-like growth factor 1 (IGF-1), and various cytokines. In order to distinguish common and unique functions of IRS-1, IRS-2, and IRS-4, we expressed them individually in 32D myeloid progenitor cells containing the human insulin receptor (32D^IR). Insulin promoted the association of Grb-2 with IRS-1 and IRS-4, whereas IRS-2 weakly bound Grb-2; consequently, IRS-1 and IRS-4 enhanced insulin-stimulated mitogen-activated protein kinase activity. During insulin stimulation, IRS-1 and IRS-2 strongly bound p85α/β, which activated phosphatidylinositol (PI) 3-kinase, protein kinase B (PKB)/Akt, and p70^s6k, and promoted the phosphorylation of BAD. IRS-4 also promoted the activation of PKB/Akt and BAD phosphorylation during insulin stimulation; however, it weakly bound or activated p85-associated PI 3-kinase and failed to mediate the activation of p70^s6k. Insulin strongly inhibited apoptosis of interleukin-3 (IL-3)-deprived 32D^IR cells expressing IRS-1 or IRS-2 but failed to inhibit apoptosis of cells expressing IRS-4. Consequently, 32D^IR cells expressing IRS-4 proliferated slowly during insulin stimulation. Thus, the activation of PKB/Akt and BAD phosphorylation might not be sufficient to inhibit the apoptosis of IL-3-deprived 32D^IR cells unless p85-associated PI 3-kinase or p70^s6k are strongly activated.