ATTEMPTS to identify an easily measured cell surface marker that would detect activation of T or B lymphocytes in a variety of species have been unsuccessful. Although insulin receptors have been identified for fat, liver, skeletal and cardiac muscle, fibroblasts and monocytes^1–8, quiescent rat splenic T lymphocytes do not bear such receptors. However, insulin receptors do emerge on T lymphocytes after concanavalin A treatment or allogeneic skin grafting^9,10. Following our work on characterising a lymphocyte insulin receptor which participates in the modulation of immune response¹¹, we have examined and report here the possibility that the emergence of a membrane-bound receptor for the peptide hormone insulin may be a universal marker for activated T and B lymphocytes.