Benzo[a]pyrene and three arene oxides of benzo[a]pyrene (benzo[a]pyrene 4,5-, 7,8-, and 9,10-oxides) have been tested for carcinogenicity in mice by topical application of each compound (0.1 or 0.4 mumol) once every 2 weeks for 60 weeks. At the high dose, benzo[a]pyrene and the 7,8-oxide were highly carcinogenic, whereas the 4,5-oxide (K-region oxide) was weakly active and the 9,10-oxide was inactive. At the low dose, only benzo[a]pyrene was highly carcinogenic. The carcinogenic activities of the three arene oxides of benzo[a]pyrene were not correlated with their stabilities or mutagenic activities.