Hydrophobic high-performance liquid chromatography of reduced and alkylated ¹²⁵I-labeled platelet-derived growth factor (PDGF) resulted in the separation of two distinct radioactive components. One of them, designated A, was resolved by polyacrylamide gel electrophoresis in sodium dodecyl sulfate, into four species with the molecular weights 18,000, 15,000, 14,000 and 11,000, respectively. The other component, designated B, showed only one major form with an M_r of about 16,000. Unlabeled PDGF yielded a similar pattern of products. A molecular model is proposed in which each molecule of PDGF consists of one A and one B polypeptide, linked by disulfide bonds. The A chain, but not the B chain, has a variable size, leading to inhomogeneity in the intact PDGF molecule (M_r 28,000 – 33,000). Some molecular weight heterogeneity is compatible with full biological activity.