Proteome analysis led to the identification and characterization of tumor-associated protein variants by two-dimensional electrophoresis and mass spectrometry. We focused on comparing the influence of genotoxic nitroso compounds N-methyl-N-nitrosourea, diethylnitrosamine and N-nitrosomorpholine and the nongenotoxic peroxisome proliferator Nafenopin as tumor-inducing agents on the protein pattern of rat hepatomas. We found several tumor-associated variants that represent members of the aldo-keto reductase superfamily. Their induction and/or inhibition was specifically related to the carcinogen used for tumor induction. The most prominent tumor-associated protein, rat aldose reductase-like protein-1 (rARLP-1) (69% sequence identity to lens aldose reductase) and three additional types of rARLP-1 were detected in nitroso compound-induced rat hepatomas, while rat aldo-keto reductase protein-c (Rak-c), a novel tumor-associated variant (65% sequence identity with 3 alpha -hydroxysteroid dehydrogenase) was discovered in N-methyl-N-nitrosourea-induced hepatomas only. 3 alpha -Hydroxysteroid dehydrogenase and delta4-3-ketosteroid-5 beta -reductase, both liver-specific enzymes, were reduced in amount in all hepatomas investigated, independent of their mode of induction. We conclude, that detoxification enzymes like 3a-hydroxysteroid dehydrogenase (3 alpha -HSD) and delta4-3-ketosteroid-5 beta -reductase (5 beta -Red) might be replaced in hepatomas by tumor-associated proteins that are often present in the embryonal state, like the rARLPs or the Rak-c protein. Their induction appears to reflect an altered constitutive pattern of detoxification enzymes, detoxifying toxic aldehydes being induced by nitroso compounds. In contrast, members of the aldo-keto reductase superfamily have not been found in Nafenopin-induced hepatomas. The pattern of tumor-associated protein variants is apparently characteristic for a given group of initiating carcinogens. The hypothesis is proposed that carcinogens leave specific fingerprints at the proteome level of manifest liver tumors.