Human Des[1-3]insulin-like Growth Factor-I (IGF-I) is a 67 amino acid analogue of human IGF-I lacking the N-terminal tripeptide Gly-Pro-Glu. Human Des[1-3]IGF-I is more potent than IGF-I in vitro and in vivo. This increased potency is due to reduced binding of human Des[1-3]IGF-I to most of the IGF binding proteins which modify the biological actions of IGF-I. Human Des[1-3]IGF-I binds to the type 1 IGF receptor with similar affinity to wild type IGF-I. This is a high quality research reagent for use in studies on cell growth, IGF receptors and IGF binding proteins.