INSULIN-LIKE growth factor-I (IGF-I) and IGF-II growth factors that are structurally related to insulin (Table 1). The human IGFs (refered to generically as somatomedins) are single chain peptides of 7.5 kilodaltons (kDa), composed of 70 and 67 amino acid residues for IGF-I and IGF-II, respectively (1–3). Four domains, designated A, B, C, and D are identified in the IGF molecules; the A and B domains being homologous with the A and B chains of insulin, respectively. In contrast with most hormonal peptides, the IGFs are secreted as they are produced. Consequently, there are no organs in which IGFs are concentrated. The liver is believed to be the principal source of circulating IGF-I (4–6) but the highest concentrations of IGFs are observed in blood (7). The IGFs are produced in most organs (8, 9) and exert biological effects on most cell types (10, 11). The ubiquity of sites of production and action has led to the concept that these peptides act by autocrine and paracrine mechanisms (12, 13) as well as by classical endocrine mechanisms (14).