Serial observations were performed on the sera α-fetoprotein (A F P) as well as on the immunohistochemical localization of A F P in liver tissues during hepatocarcinogenesis induced with 3'-Me-DAB in rats. Results showed that the dynamics of the sera A F P in rats bearing hepatocellular carcinomas (HCC) revealed saddle-like curve, whereas that of those rats without hearing HCC only increased at a sustained low level. The pathological bases which are responsible for precancerous increase of sera AFP are as follows: the small basophilic hepatocytes, "survival" hepatocytes, hyperplastic nodules, dysplastic hepatocytes, the hepacytic cords within the cholangiofibrosis foci. Solt-Farber model analysis indicated that the increase of sera AFP was related to the proliferation of the initiated cells. Immunoelectronic microscopy demonstrated the AFP was located on the membranes, which may be caused by the absorption of AFP from blood onto the cell membranes.