The association of somatomedin (Sm) peptides with their specific serum binding proteins (SmPBs) and the preservation of SmBP integrity are both pH dependent. Acid extracts of human plasma Cohn fraction IV-4 chromatographed at pH 5.0 after incubation with [125I]iodoinsulin-like growth factor I yield predominantly a 60,000 molecular weight complex of specifically bound radiolabeled peptide. Alkaline exposure (pH 8.0) of either the initial acid extract of Cohn fraction IV-4 or the isolated 60,000 molecular weight chromatographic peak shifts the recovery of bound [125I]iodoinsulin-like growth factor I on rechromatography to two smaller complexes of approximately 46,000 and 30,000 molecular weight. These results support the existence of two or more forms of human plasma SmBP that may possess a common 30,000 molecular weight component.