Cultured human lymphocytes and rat hepatoma cells were labeled with [32P]orthophosphate and the insulin receptor subunits identified by immunoprecipitation and sodium dodecyl sulfate-gel electrophoreses. In both cell types the 95,000-dalton (beta) subunit of the insulin receptor was selectively phosphorylated. Phosphorylation was specifically stimulated by insulin in a dose-dependent fashion after 1 and 15 minutes of hormone treatment, whereas human growth hormone was without effect. This phosphorylation may be a very early event in insulin action.