Reduced glucose-6-phosphatase, increased GGT activity and reduction of cytochrome P-450 content are considered to be markers of chemical hepatocarcinogenesis in rats. The significance of these changes were studied in certain human liver lesions; adenoma, focal nodular hyperplasia and hepatocellular carcinoma all developed in non-cirrhotic livers. Enzymes showed normal values in 4 out of 5 adenomas, in 2/13 FNH and in 4/18 HCC samples. The decreased cP-450 content in HCC proved to be the most consistent alteration (12/18). Only 3 HCC samples possessed changes off all enzymes. These data suggest that at least those enzymes which are used as markers in rat chemical hepatocarcinogenesis have little or no biological significance in human liver tumors, primarily due to the intertumoral heterogeneity of enzyme activity. Such heterogeneity was observed in the peritumoral "normal" liver tissue, too.