The insulin-like growth factor (IGF)-binding proteins present in the human serum and various biological media have been characterized by several methods: gel filtration, sucrose gradient sedimentation, polyacrylamide gel electrophoresis and chromatofocusing. Several forms have been identified with molecular weights of ∼ 42,000, 39,000, 34,000, 30,000 and 24,000 daltons. Results of competitive binding studies suggest that the different forms of binding proteins have different affinities for IGF-I and IGF-II. The influence of various hormones and pathophysiological conditions on the biosynthesis of the binding proteins has been investigated.