Four structural analogs of human insulin-like growth factor I (hIGF-I) have been prepared by site-directed mutagenesis of a synthetic IGF-I gene and subsequent expression and purification of the mutant protein from the conditioned media of transformed yeast. [Phe-1,Val1,Asn2, Gln3,His4,Ser8, His9,Glu12,Tyr15,Leu16]IGF-I (B-chain mutant), in which the first 16 amino acids of hIGF-I were replaced with the first 17 amino acids of the B-chain of insulin, has greater than 1,000-, 100-, and 2-fold reduced potency for human serum binding proteins, the rat liver type 2 IGF receptor, and the human placental type 1 IGF receptor, respectively. The B-chain mutant also has 4-fold increased affinity for the human placental insulin receptor. [Gln3,Ala4]IGF-I has 4-fold reduced affinity for human serum binding proteins, but is equipotent to hIGF-I at the types 1 and 2 IGF and insulin receptors. [Tyr15,Leu16]IGF-I has 4-fold reduced affinity for human serum binding proteins and 10-fold increased affinity for the insulin receptor. This peptide is also equipotent to hIGF-I at the types 1 and 2 IGF receptors. The peptide in which these four-point mutations are combined, [Gln3,Ala4,Tyr15,Leu16]IGF-I, has 600-fold reduced affinity for the serum binding proteins. This peptide has 10-fold increased potency for the insulin receptor, but is equipotent to hIGF-I at the types 1 and 2 IGF receptors. All four of these mutants stimulate DNA synthesis in the rat vascular smooth muscle cell line A10 with potencies reflecting their potency at the type 1 IGF receptor. These studies identify some of the domains of hIGF-I which are responsible for maintaining high affinity binding with the serum binding protein and the type 2 IGF receptor. In addition, these peptides will be useful in defining the role of the type 2 IGF receptor and serum binding proteins in the physiological actions of hIGF-I.