Insulin potentiated the dextran anaphylactoid reaction in untreated rats and in rats pretreated with a protein synthesis inhibitor or with a lymphopenic agent. Incubation of rat lymphatic tissue with insulin failed to produce a factor potentiating the histamine-releasing action of dextran when this was studied in both in vivo and in vitro tests. It thus seems that insulin is exerting its potentiating effect on the dextran response through processes not involving synthesis of new protein or interaction with lymphocytes.