The expression of insulin-like growth factor (IGF) receptors at the cell surface and the changes in IGF responsiveness during differentiation were studied in the L6 skeletal muscle cell line. Throughout the entire developmental sequence, distinct receptors for IGF I and IGF II that differed in structure and peptide specificity could be demonstrated. During differentiation, both 125I-IGF I and 125I-IGF II binding to the L6 cells decreased as a result of a 3-4-fold reduction in receptor number, whereas 125I-insulin binding increased. Under nonreducing conditions, disuccinimidyl suberate cross-linked 125I-IGF I and 125I-IGF II to two receptor complexes with apparent Mr greater than 300,000 (type I) and 220,000 (type II). Under reducing conditions, the apparent molecular weight of the type I receptor changed to Mr 130,000 (distinct from the 120,000 insulin receptor) and the type II receptor changed to 250,000. IGF I and IGF II both stimulated 2-deoxy-D-glucose and alpha-aminoisobutyric acid uptake in the L6 cells with a potency close to that of insulin, apparently through interaction with their own receptors. The stimulatory effects of IGF II correlated with its affinity for the type II but not the type I IGF receptor, as measured by inhibition of affinity labeling, whereas the effects of IGF I correlated with its ability to inhibit labeling of the type I receptor. In spite of the decrease in type I and type II receptor number, stimulation of 2-deoxy-glucose and alpha-aminoisobutyric acid uptake by the two IGFs increased during differentiation.