To identify structural characteristics of the closely related cell surface receptors for insulin and IGF‐I that define their distinct physiological roles, we determined the complete primary structure of the human IGF‐I receptor from cloned cDNA. The deduced sequence predicts a 1367 amino acid receptor precursor, including a 30‐residue signal peptide, which is removed during translocation of the nascent polypeptide chain. The 1337 residue, unmodified proreceptor polypeptide has a predicted Mr of 151,869, which compares with the 180,000 Mr IGF‐I receptor precursor. In analogy with the 152,784 Mr insulin receptor precursor, cleavage of the Arg‐Lys‐Arg‐Arg sequence at position 707 of the IGF‐I receptor precursor will generate alpha (80,423 Mr) and beta (70,866 Mr) subunits, which compare with approximately 135,000 Mr (alpha) and 90,000 Mr (beta) fully glycosylated subunits.