Early studies and reviews of the effects of carcinogens on bacteria and yeast are noted, after which a brief discussion deals specifically with chemical carcinogens and their relations with mutagenesis, deficiencies in DNA repair, RNA synthesis and function, and respiration‐deficient mutants. Specific chemical carcinogens are then reviewed as to their mode of action, mutagenic and carcinogenic potency, and biochemical effects, including induced biological alterations such as cytogenetic varients. Structure‐function relations are discussed for some types of compounds. The importance ofN‐hydroxylation of parent amines for mutagenicity and its effect on carcinogencity are related. Epoxide derivatives of polycyclic hydrocarbons can become covalently bound to nucleic acids. Nitrofurans have an excellent correlation with respect to their mutagenicity and carcinogenicity. Of theN‐oxides, 4‐nitroquinoline‐1‐oxide and its hydroxy derivative have been studied most extensively in a variety of bacterial systems regarding their mode of action. Acridine intercalation of DNA and covalent binding to nucleic acids of acetylaminofluorene after acetoxy derivative formation are reactions involved in the mutagenicity of bacteria and carcinogenicity in higher forms. β‐Propiolactone and nitrosamines acting as alkylating agents can cause frameshift mutations, as can aflatoxins, but only after metabolic conversion. Alkylating agents per se are mutagenic and carcinogenic without metabolic conversion. Ethionine, a carcinogenic agent, is nonmutagenic but can induce certain enzyme activities in yeast cells similar to those found in tumor cells. Several other kinds of chemical carcinogens are also reviewed.