Chemical hepatocarcinogenesis in female mice, induced by 5,9-dimethyl(7H)dibenzo[c,g]carbazole, leads to the overexpression of a cytochrome P-450 of the 2a family. This protein was identified as Cyp2a-5, by the use of immunoblots obtained from isoelectric focusing gels. This method allowed the distinction of Cyp2a-5 from Cyp2a-4, another mouse liver cytochrome P-450, by taking advantage of their slightly different pI values. The theoretical pI values, determined from the amino acid sequence, were pI 9.91 for Cyp2a-4 and pI 10.01 for Cyp2a-5. Other structurally related forms were not detected. In hepatomas from female mice, only the Cyp2a-5 form was overexpressed (2-3 fold). Male mice showed a weak expression of Cyp2a-4 and Cyp2a-5 in control liver samples and in hepatomas. The expression of both forms was increased more than fivefold upon castration. Pyrazole induces specifically the Cyp2a-5 form. The Cyp2a-5 overexpression was correlated with enhanced microsomal coumarin-7-hydroxylase and testosterone-15 alpha-hydroxylase activities. An immunohistochemical study showed that Cyp2a-4 and Cyp2a-5 are expressed uniformly in female livers, but centrilobularly in male livers. In hepatomas, this localisation is perturbed; in females we observed a focal cell localisation, and the Cyp2a-containing cells were often hypertrophic and polyploid. In hepatomas from male mice, the Cyp2a-containing cells became dispersed. From a comparison with other studies, the Cyp2a-5 overexpression appears to be a general feature of hepatocarcinogenesis in mice.