To purify a smooth muscle cell derived growth factor,the serum free medium conditioned by cultured rabbit aortic smooth muscle cells(SMC-CM)was collected.Methods The SMC-CM was 5.8 fold concentrated by ultrafiltration using Millipore Ultrafiltration System with a molecular weight cut off at 10 kDa,and then was purified by heparin affinity chromatogrphy using a column of heparin-Sepharose CL-6B.Incorporation of3H-thymidine(3H-TdR)into cell DNA was used to measure the mitogenic activity of the fractions from chromatography for NIH 3T3 fibroblasts. The molecular weight and the iso-electric point of these fractions were determined by NaDodSO4-polyacrylamide gel electrophoresis (SDS-PAGE) and iso-electric focusing, respectively. Results The protein eluted in 1.0～1.6 mol·L-1 NaCl from the heparin-sepharose was mitogenic for 3T3 cells, and this protein had a molecular weight of 23.0～26.9 kDa and iso-electric point of about 4.6. Conclusions The fact that the above-mentioned biochemical properties differed from that of PDGF, IGF and FGF suggests that this mitogenic protein may be a separate growth factor.