We have examined the effect of cholera toxin (CT) on the insulin receptor tyrosine kinase. Incubation of intact rat hepatoma cells FaO with CT (1 microgram/ml/2h) inhibited insulin-induced receptor autophosphorylation by 30% in vivo. This effect persisted after receptor purification in vitro. CT did not alter hormone binding of the insulin receptor, indicating that the toxin affects signal transduction of insulin at the level of the receptor kinase. Experiments using chinese hamster ovary (CHO) cells transfected either with the human insulin receptor (HIR) or a mutant lacking the last 43 amino acids of the receptor beta-subunit (HIR delta CT) showed, that the carboxy-terminal tail of the insulin receptor does not play a role in the suppressive effect of the toxin on the insulin receptor kinase.