The acceleration of tumor production in mice by repeated application of polycyclic aromatic hydrocarbon carcinogens of high and low potency in the presence of cocarcinogenic compounds is described. Experiments exploring the concentration levels of carcinogen and cocarcinogen necessary to produce tumors demonstrate that there is a 1,000-fold increase in the enhancement of potency of low concentrations of benzo[a]pyrene and benz[a]anthracene when n-dodecane is the diluent. The effect of cocarcinogens is most obvious during exposure to low concentrations of carcinogen. The implications of this observation in the environment is discussed.