Testing of chemicals for carcinogenic activities is an important part of a comprehensive toxicological test program. Screening tests (short-term tests) can be utilized as methods to select suspicious chemicals which should or must be further tested in long-term animal experiments. Up to now long-term animal bioassays, even when taking into account all limitations of evaluation and extrapolation of results to man, rank prominent in predicting carcinogenicity of a compound in man. Any substance which is shown to cause tumors in animals should be considered carcinogenic and therefore a potential hazard for man.