Human platelet-derived growth factor (PDGF) is a potent mitogenic polypeptide which is believed to be a heterodimer of A- and B-chains stabilized by interchain disulphide bonds. The B-chain of PDGF is encoded by the c-sis gene, the normal cellular homologue of the transforming gene of the simian sarcoma virus (SSV). cDNA clones of the B-chain from both normal and transformed cells have mutually consistent DNA sequences. Recently, an A-chain cDNA clone (D-1) was isolated from a transformed human glial cell cDNA library. We report the complete sequence of an A-chain cDNA clone (BT-1) isolated from a normal human umbilical vein endothelial (HUVE) cell cDNA library. BT-1 differs from the sequence of the D-1 clone by a 69 base pair deletion containing the predicted carboxy terminus of the protein. The mRNA levels of the A- and B-chains of PDGF in HUVE cells were analysed and shown to respond differently to the endothelial cell growth factor (ECGF).