The carcinogenic (on mice) and mutagenic (on bacteriaSalmonella typhimuriumTA-98 and TA-100) action of benz(a)pyrene (BP) and its derivatives: 6-methyl-, 6-formyl-, 6-chloro-, 6-hydroxy-, 6-acetoxy-, 6-methoxy-, and 4 (5)-methoxy-BP was investigated. Powerful carcinogens (BP, 6-methyl- and 6-formyl-BP) were shown to cause mutations in both strains of bacteria. Weakly carcinogenic compounds [6-chloro-, 6-methoxy-, and 4(5)methoxy-BP] and noncarcinogenic (6-acetoxy- and 6-hydroxy-BP) compounds either were nonmutagenic or induced mutations in bacteria of only one of the two strains. Differences between the carcinogenic and mutagenic actions of the various compounds were not related to the velocity of their oxidation in an enzymic system.