Evidence from radioligand assays that somatomedin-C and insulin-like growth factor-I are similar to each other and different from other somatomedins.

Somatomedin-C (SM-C) and insulin-like growth factor I (IGF-I) produced identical curves of competition in a RIA for SM-C, and in placental cell membrane receptor assays for SM-C and insulin. Somatomedin-A (SM-A), insulin-like growth factor II (IGF-II) and two forms of multiplication stimulating activity (MSA) were less than 5% as potent as SM-C/IGF-I in the RIA and less than 50% that of SM-C/IGFi in the SM-C receptor assay. In the insulin receptor assay, IGF-II and MSA III-2 were much more potent than SM-C/IGF-I. The present data suggest that SM-C and IGF-I are functionally identical substances.