The stability of cytokeratin during tumor transformation in hepatocellular carcinoma was studied. We applied biochemical methodology to look into the switching of cytokeratin molecules in tumor transformation. First, by centrifugation the cytokeratin molecules were extracted from both liver and hepatoma tissues. The extracts were then soaked with cyanogen bromide-activated Sepharose 4B beads previously coated by monoclonal anti-cytokeratin antibody. The bound molecules were then released from the resin with salt. Second, the isolated molecules of both were treated with lysosomal enzyme and analyzed on two-dimensional gels. The results demonstrated that there was a modulation in cytokeratin molecules, and the hepatoma cytokeratin was generated from the hepatocyte cytokeratin.