We have carried out an investigation to determine whether or not the sister-chromatid exchange frequencies (SCEs) observed in bone marrow cells in mice treated with mitomycin C (MMC) are inhibited by the immunopotentiators Krestin and Lentinan. We found that mitomycin C (2 mg/kg, i.v.)-induced SCEs were inhibited in 27% of the mice treated with Krestin (300 mg/kg, i.p.) and in 23% of the mice treated with Lentinan (1 mg/kg, i.p.). The effects of Krestin were found to be dose-dependent in inhibition of MMC-induced SCEs while those of Lentinan were not. Our findings therefore suggest that Krestin and Lentinan are not only useful for cancer treatment as immunopotentiators in combination with anticancer drugs but may also prevent the increase of chromosomal damage induced by anticancer drugs.