CHO cells were transfected with plasmid pSM-1 (containing human c-sis cDNA) singly or co-transfected with pSV 2 neo DNA by calcium phosphate method. After low serum or G418 selection several cell lines with expression of platelet-derived growth factor (PDGF) were obtained. One among them, FB5, was of the highest PDGF expression and showed the following biological characteristics when compared with CHO cells: (1) a prominent change in morphology from spindle to round in shape: (2) increase of growth rate; (3) growth in low serum (2%) medium as a semisuspension culture; (4) growth on soft agar to larger colonies; (5) synthesis of PDGF in cytoplasm identified by immunofluorescent method; (6) the conditioned medium stimulated DNA synthesis of NRK cells; (7) RNA dot hybridization showing high transcription of PDGF mRNA; (8) southern blot showing integration of human c-sis gene was still stable after 7 months. These results indicated that intergration of exogenous c-sis gene and its high expression might cause CHO cells to high growth rate and even transformation. The establishment of this stable transformed cell line, FB5 is thought to be a good model for further study on the function of PDGF in cell growth control and cell transformation.