Platelet-derived growth factor (PDGF) is a dimeric protein composed of A- and B-chains (AA, AB, and BB). PDGF purified from human platelets has been shown to be composed primarily of the AB heterodimer. Immunoblots of total platelet extracts, cell-bound PDGF from the platelet extracts, and acute human wound fluid with PDGF A- and B-chain-specific antisera all demonstrate that the PDGF A-chain is the predominant peptide. Chemotactic and immunochemical assays of chromatographic fractions during PDGF isolation support these observations and demonstrate that PDGF AA can be separated from PDGF AB and BB by ion exchange chromatography. These studies indicate that the AA isoform constitutes the major PDGF dimer contained in human platelets and is the major form present at sites of injury during the acute phase of the wound repair response.