The effect of fractions from a water extract of Polyporus on bladder tumor promotion was examined using 5% sodium saccharin (SS) in a short-term test with concanavalin A (Con A) in Wistar rats. Rats were given N-butyl-N-(4-hydroxybutyl) nitrosamine (BHBN) in drinking water for one week, and then promoter alone or test samples (given orally) plus promoter was administered for 3 weeks. Treatment with the BuOH fraction isolated from the water extract showed a strong inhibitory effect against the promoter. It was found that the inhibitory effect of the BuOH fraction is due to the effect of ergosterol contained in the fraction. Treatment with ergosterol showed a strong inhibitory effect against 5% SS, 0.01% BHBN, 3% DL-tryptophan (Trp) or 2% butylated hydroxyanisole (BHA); ID50 was 1.4 microg/kg/d, 2.9 microg/kg/d, 11.6 microg/kg/d, and 11.7 microg/kg/d against SS, BHBN, Trp and BHA, respectively. We also examined the effect of steroids and related compounds. Squalene and vitamin D2 showed strong inhibitory effect against 5% SS-induced bladder tumor promotion. These results strongly suggest that ergosterol could provide significant protection against the promotion of bladder tumor induced by many types of promoters in the environment.