Up to now, the carcinogenicity of a substance, i.e., its cancer-causing effect, has been usually determined with the help of extensive animal testing. The in ovo carcinogenicity assay (IOCA) has been suggested as a rapid and inexpensive, non-animal, method for carcinogenicity testing and for experimental studies on mechanisms of carcinogenesis. The substance to be tested is injected into a fertilized ovum. No later than four days before the hatching date, the embryos are released, and the liver is removed for identifying the effect of hepatocarcinogens. Histological, cytological and molecular biological alterations of the embryonic liver have been induced with a variety of chemical carcinogens. After sufficiently high doses of hepatocarcinogens, tubular structures predominate in the liver and replace the normal trabecular pattern. The cell and nuclear size of the hepatocytes in embryonic liver is severely increased after exposure to chemical carcinogens over a wide dose range including doses that fail to elicit cytotoxicity in the embryonic liver.