FOY-305 is a synthetic serine protease inhibitor and ONO-3403 is a derivative with a higher protease-inhibitory activity. The growth-suppressive effects of ONO-3403 were more prominent in Ha-ras-transformed NIH3T3 (ras-NIH) cells than in non-transformed NIH3T3 cells. After treatment of ras-NIH cells with ONO-3403 at 100-200 microg/ml, the percentage of cells found in G(1) phase decreased and, concomitantly, that in S phase increased. Molecular events caused by ONO-3403 were investigated by Western blot analysis using anti-phosphotyrosine antibody. The results showed a marked decrease in the tyrosine phosphorylation level of a 180-kDa protein after treatment with ONO-3403. This 180-kDa phosphotyrosine-containing molecule which was tentatively designated pY-p180 might be platelet-derived growth factor (PDGF) receptor since addition of PDGF to serum-starved NIH3T3 cells induced a marked tyrosine phosphorylation of the same size within 5 min. This was further confirmed by immunoprecipitation of cell extract with anti-PDGF-receptor antibody followed by Western blot analysis using anti-phosphotyrosine antibody. Treatment of T.Tn human esophageal carcinoma cells with ONO-3403 caused also decrease in pY-p180, which appeared to be epidermal growth factor receptor. Thus, ONO-3403 may induce growth suppression by down-regulation of cell surface growth factor receptors.