Interchain cystines of PDGF-BB dimer were characterized by Edman reaction and by SDS-PAGE analysis on the protein which was chemically cleaved at Trp-40. It was found that Cys-43 has a key role in dimer formation, asymmetrically cross-linked to a cysteine residue of another identical subunit. The remaining cystines participate in the intramolecular disulfide linkages. Pepsin digestion of PDGF-BB dimer generated several small peptides and one ubiquitous Cys-containing peptide. Sequence analyses of several Cys-containing peptides indicated the existence of three intramolecular disulfide linkages including Cys-16--Cys-60, Cys-49--Cys-97, and Cys-53--Cys-99. Two interchain disulfide bonds of Cys-43--Cys-52 between two subunits were deduced from the partial reduction and alkylation of PDGF-BB. This study provides chemically determined disulfide linkages of PDGF-BB.