Abstract: AIM　To study the antiproliferative effect of heparinoid derived from porcine duodenum (heparinoid) on cultured vascular smooth muscle cells. METHODS　Cultured bovine aortic smooth muscle cells (BASMCs) of 5～10 passages were seeded into 24 and 72-well cluster culture plates and were synchronized by 48 h serum deprivation. Then, the cells were re-stimulated by serum repletion with or without heparinoid. The antiproliferative effect of heparinoid was evaluated by crystal violet staining and MTT assay 72 h after serum repletion. To study the drug action on cytomorphologial changes, three kinds of cells ［quiescent cells, cells treated with 10% fetal calf serum (FCS) with or without heparinoid］ were observed by transmission electron microscopy. After synchronized and re-stimulated as above, BASMCs were treated with heparinoid 0.8 mg.mL^-1 at selected points during serum repletion. The cells were harvested at specified times after serum repletion, then cellular DNA contents (to estimate the proportions of cells in different phases of the cell cycle) and the contents of α-actin, c-myc and c-fos proteins were measured by flow cytometry. RESULTS　Heparinoid was shown to inhibit the proliferation of BASMCs induced by 10% FCS. The inhibitory effect was weakened when heparinoid was added 2 h after serum repletion, and there was no antiproliferative effect when heparinoid was added 12 h after serum repletion. Electron micrographs showed that cells treated with 10% FCS and heparinoid expressed a contractile phenotype, while cells treated with 10% FCS only expressed a synthetic phenotype. Flow cytometry study showed remarkable increase of α-actin, and decrease of c-myc and c-fos proteins in the cells treated with heparinoid. CONCLUSION　Heparinoid was found to inhibit the proliferation of BASMCs. The antiproliferative effect occurred at the early phase of the cell cycle. It might be due to the drug′s influence on cell phenotype modulation and the down regulation of c-myc and c-fos proto-oncogenes expression.