Somatomedin and peptides are bound tightly to a plasma complex in vivo. There are at least two macromolecular forms of somatomedin complexes: one with a molecular weight of 150 000 the ('150 K' complex) and one of 45 000 (the '45 K' complex). These macromolecular complexes clearly increase the half-life of somatomedins in plasma. The 150 K complex appears to have a three-subunit structure consisting of the somatomedin peptide, a specific Sm/IGF binding protein, and an acid-labile component. In addition to the binding proteins contained in the 150 K and 45 K complexes, plasma also contains unsaturated Sm/IGF binding proteins of approximately 40 000 molecular weight. The somatomedin complex forms not only have longer half-lives, but in this form their biological actions (both insulin-like and growth-promoting) are clearly modified. In addition, it is possible that the complex forms play a role in tissue selectivity and/or delivery of active somatomedin peptide to the intracellular space. An understanding of the somatomedin complexes, plasma complexes and binding proteins is necessary before the physiological roles of the somatomedin peptides can be defined more clearly.