Platelet-derived growth factor (PDGF) is a basic protein of relative molecular mass 30,000 (Mr 30K) composed of two polypeptide chains, designated PDGF A and PDGF B. The B-chain is encoded by the c-sis gene, the cellular counterpart of the simian sarcoma virus transforming gene v-sis. The PDGF A-chain cDNA clones recently isolated and sequenced from a transformed human clonal glioma cell line represent at least two alternatively spliced transcript species differing by 69 base pairs at the C-terminus. Here we demonstrate that the normal human umbilical vein endothelial cell (EC) A chain precursor lacks the 15 carboxy-terminal, highly basic amino acids encoded by the larger tumour cell cDNA. Surprisingly, culture media from monkey kidney cells (COS) transfected with the endothelial cDNA clone contained much less mitogenic activity than media from cells transfected with the longer tumour cell-derived A-chain cDNA. This functional difference appeared to be due to inefficient assembly or secretion of the recombinant endothelial-type growth factor. This suggests that some transformed cells may use alternative RNA splicing to modify normal growth factors and by so doing increase the efficiency of mitogen assembly or secretion.