Abstract. Binding of 125I-insulin-like growth factor-I and 125I-ovine somatomedin-C/IGF-I to monolayer cultures of muscle-derived ovine fibroblasts is described. Preliminary competitive binding experiments indicate that ovine fibroblasts possess independent cell surface receptors for IGF-I. Affinity of rIGF-II for IGF-I binding sites is minimal; rIGF-II binds to Type I IGF receptors at 1/1000 the strength of IGF-I. Insulin binds to the Type I IGF receptor at 1/100 the strength of IGF-I, whereas ovine somatomedin-C/IGF-I displays equivalent IGF-I binding as evidenced by overlapping competition of ovine somatomedin-C/IGF-I for 125IIGF-I binding sites. Results from disuccinimidyl suberate cross-linking of 125I-IGF-I to muscle-derived ovine fibroblasts in the presence of related polypeptides verified the competitive binding data. Under reducing conditions, 125I-IGF-I: receptor complexes migrated to a relative molecular weight of approximately 135 000 daltons. Specific 125I-IGF-I binding was completely inhibited by 10−8 mol/l IGF-I, 7.2 × 10−8 mol/l ovine somatomedin-C/IGF-I, and 10−6 mol/l insulin and partially inhibited by 7.2 × 10−9 mol/l ovine somatomedinC/IGF-I and 6.5 × 10−8 mol/l rIGF-II. 125I-ovine somatomedin-C/IGF-I: receptor complexes also migrated at a relative molecular weight of 135 000 daltons. No migratory band was observed at 250 000 to 260 000 daltons with either 125I-IGF-I or 125I-ovine somatomedin-C/IGF-I indicating that little labelled moiety bound to the Type II IGF receptor. Based on these preliminary competitive binding studies and cross-linking data, we conclude that ovine somatomedin-C/IGF-I is primarily interacting with the Type I IGF membrane receptor on ovine skeletal muscle fibroblasts.