The insulin receptor and type 1 insulin-like growth factor (IGF) receptor as classically described are each the product of a single gene. Various receptor subtypes have been described, however, with distinct structures or binding properties. Two of these subtypes have been studied, namely hybrid and atypical IGF-I receptors. Hybrid receptors contain aβ halves of both the insulin and the IGF receptor. They are identifiable as a high-affinity IGF-I-binding species reacting with both IGF-receptor-specific and insulin-receptor-specific monoclonal antibodies, and account for a substantial fraction of IGF receptor in many mammalian tissues. Hybrid receptors purified from human placenta bind IGF-I with approximately 25-fold higher affinity than insulin, the affinity for insulin being 10-fold less than that of the classical insulin receptor. It is therefore likely that hybrids will respond more readily to IGF-I than to insulin in vivo. Atypical IGF receptors are characterized by an ability to bind insulin as well as IGFs with relatively high affinity, but are immunologically indistinguishable from classical IGF receptor and do not react with insulin receptor-specific antibodies. The structural basis of atypical binding behaviour is unknown, though the effect is mimicked by binding of certain anti-IGF receptor monoclonal antibodies, which dramatically increase the affinity of the IGF receptor for insulin. Specific physiological roles have not been demonstrated for hybrid or atypical receptors, but the available information concerning their distribution and properties suggests that these receptor subtypes may have an important influence on the specificity of action of insulin and IGFs in vivo.