4'-Aminomethyl-4,5',8-trimethylpsoralen has been chemically conjugated to insulin using a carbodiimide derivative. The psoralen moiety retains its photochemical reactivity as evidenced by its ability to crosslink DNA after exposure to long wavelength ultraviolet light (UVA, 320-400 nm). This chimeric molecule has been used to selectively kill a population of lymphocytes whose expression of insulin receptors has been stimulated with phytohemagglutinin. Insulin carries the psoralen into the cell via receptor-mediated endocytosis, where it is subsequently activated by exposure to UVA light. The UVA induced activity of AMT-insulin can be blocked by the presence of native insulin. The viability of unstimulated lymphocytes was not affected by AMT-insulin and UVA light. The hybrid insulin-psoralen molecule may be a prototype for a family of phototoxic drugs which can be selectively delivered to subsets of lymphocytes.