The surface topography of IGF I(insulin-like growth factor I) was investigated by chemical modification of amino acid residues in free IGF I and bound to type I IGF receptor or to monoclonal antibody MAB43. Tyrosine residues were modified either by chloramine-T or lactoperoxidase catalyzed iodination. In the free IGF I molecule, all 3 tyrosine residues, A19 (Tyr-60), B25 (Tyr-24), and C2 (Tyr-31), were iodinated. Monoclonal antibody MAB43 protected IGF I against modification at tyrosine residue A19, and in the type I IGF receptor-IGF I complex, all 3 tyrosine residues were shielded against iodine incorporation. These results allow the prediction of the binding domains in the IGF I molecule. The minimal receptor binding site in IGF I would include amino acid residues B25 to C2 and, possibly, the C-terminal part of the A-domain with tyrosine residue A19.