Inhibition bioassay of sister chromatid exchanges (SCE) and DNA synthesis was carried out to study the mechanism that selenium, as a chemical agent for intervention, could antagonize the effects of arsenic on body damage and lower incidence of tumor. Results demonstrated that SCE frequency in peripheral lymphocytes induced by As2O3 could be significantly reduced by preincubation with 5 x 10(-7), 1 x 10(-6) mol/L Na2SeO3 and co-exposure of 1 x 10(-6) mol/L selenomethionine. Inhibition of DNA synthesis could be antagonized by preincubation in with 5 x 10(-6) mol/L Na2SeO3 and co-exposure of 1 x 10(-6), 5 x 10(-6) mol/L selenomethionine. But, the mechanism that protective effects of selenium on the damage to cell genetic material induced by arsenic should be studied further.