Our laboratory have found that human aortic heparan sulfate proteoglycan(HSPG) can suppress the proliferation of cultured fetal aortic smooth muscle cells(hASMC).In this study Norther blot was used to invetigate the effect of HSPG on the expression of c-fos and c-myc proto-oncogene in hASMC. The results showed that former can suppress both c-fos and c-myc gene. It is concluded that suppression of protooncogene expression may be one of the causes that HSPG suppresses smooth muscle cell proliferation.