We identified SH2-Bα as an insulin-receptor-binding protein based on interaction screening in yeast hybrid systems and co-precipitation in cells. SH2-Bα contains pleckstrin-homology (‘PH’) and Src homology 2 (SH2) domains and is closely related to APS (adapter protein with a PH domain and an SH2 domain) and lnk, adapter proteins first identified in lymphocytes. SH2-Bα is ubiquitously expressed and is present in rat epididymal adipose tissue, liver and skeletal muscle, physiological sites of insulin action. On SDS/PAGE, SH2-Bα migrates at a molecular mass of 98 kDa, although the predicted size of SH2-Bα is 79.6 kDa. Insulin causes an electrophoretic mobility shift. SH2-Bα can be immunoprecipitated using anti-(insulin receptor) antibody from insulin-stimulated cells. Anti-phosphotyrosine antibody or the growth factor receptor-binding protein 2 (Grb2) SH2 domain precipitate SH2-Bα after insulin stimulation, suggesting that SH2-Bα is tyrosine-phosphorylated and may be a substrate for the insulin receptor. The SH2-Bα SH2 domain did not interact with insulin-receptor substrate (IRS) proteins or epidermal-growth-factor receptor. Mutation of the juxtamembrane and C-terminus of the insulin receptor did not abolish the interaction with the SH2 domain. This was further confirmed using a panel of activation-loop single point mutants where mutation of Tyr1158, Tyr1162 and Tyr1163 abolished interaction. Thus SH2-Bα is a likely component in the insulin-signalling pathway and may function as an alternative signalling protein by interacting with the activation loop of the insulin-receptor cytoplasmic domain.