Inhibitory activity against 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced inflammation in mice was observed in the methanol extract of Chlorella vulgaris, a green alga. The hexane soluble fraction obtained from the methanol extract exhibited marked inhibitory activity from which were isolated two delta(5,7)-sterols (ergosterol and 7-dehydroporiferasterol), two delta(5,7,9(11))-sterols [7,9(11)-dehydroergosterol and 7,9(11)-bisdehydroporiferasterol], two 5 alpha, 8 alpha-epidioxy-delta(6)-sterols (ergosterol peroxide and 7-dehydroporiferasterol peroxide), and a 7-oxo-delta(5)-sterol (7-oxocholesterol), among others. The delta 5'7-sterols, 5 alpha, 8 alpha-epidioxy-delta(6)-sterols and 7-oxo-delta 5-sterol inhibited TPA-induced inflammation in mice. The 50% inhibitory dose of these compounds for TPA-induced inflammation was 0.2-0.7 mg/ear. Furthermore, ergosterol peroxide markedly inhibited the tumor-promoting effect of TPA in 7,12-dimethylbenz[a]anthracene-initiated mice.