Rat insulin-like growth factor-I (IGF-I) is a single chain 70 amino acid polypeptide that shares structural homology with human and rat IGF-II. Only three amino acids are different compared to the sequence of human IGF-I. Rat IGF-I has been shown to stimulate growth in L6 myoblasts and other cell lines. Receptor Grade rat IGF-I is a high quality research reagent particularly for use in studies on cellular and animal growth, for use in specific assays for rat IGF-I and other general research purposes.