Previous study of rat liver during chemically induced hepatocarcinogenesis has shown that expression of isoforms of the 27-kD heat-shock protein was greater in neoplastic nodules and in hepatocellular carcinoma than in control livers. In this study, various human neoplastic and nonneoplastic liver tissues were investigated with electrophoresis after amino acid labeling to evaluate the expression of 27-kD heat-shock protein isoforms. This revealed that human liver contains 27-kD proteins that are recognized by a polyclonal antibody raised against human 27-kD heat-shock protein. Basal levels of fluorographical and immunostaining intensity of the 27-kD heat-shock protein spots (respectively, after [3H]leucine or 32P incorporation or as checked with a specific human 27-kD heat-shock protein antibody) were higher in hepatomas than in non-tumorous liver. Phosphorylation patterns of the 27-kD heat-shock protein isoforms were, however, similar in hepatocellular carcinoma and in uninvolved surrounding liver. Heat inducibility of the 27-kD heat-shock protein, tested in one case of liver cell adenoma and in the surrounding liver, was also preserved in both tissues. The role of the overexpression of 27-kD heat-shock protein in neoplastic liver tissues remains unknown. We propose, as a working hypothesis, that it is related to the resistant phenotype acquired by some tumors during malignant progression. (HEPATOLOGY 1992;16:382–389.)