Using a human fetal lung-derived diploid fibroblast, HAIN-55, the effect of oxidizing agents such as vitamin K (VK) and lipid peroxide on cell proliferation was examined. When the cells in G₀ period were treated with these oxidizing agents, the proliferation of cells and DNA duplication induced by serum were inhibited. The appearance of competence gene mRNA such as c-fos and c-myc induced in the cells on addition of platelet-derived growth factor (PDGF) was also suppressed on the treatment with oxidizing agents. All these inhibitions were eliminated by re-addition of SH-protectors such as dithiothleitol. According to the results of these experiments, oxidizing agents appear to block the stream of signal transduction through oxidative modification of SH-groups of some proteins during the process of competence, thereby inhibiting the transition from G₀ to G₁ and finally to S stage and this would result in a suppression of cell proliferation.