This annual report provides a list of all substances either known or anticipated to be carcinogens to which a significant number of persons living in the United States are likely to be exposed. The report lists 148 agents, including the list from the Third Annual Report plus 31 additional substances or groups of substances. Of the 148 entries, 24 substances or groups of substances and five occupational exposures associated with a technological process were listed as known carcinogens. These substances include 4-aminobiphenyl (92671), analgesic mixtures containing phenacetin (62442), arsenic (7440382) and certain arsenic compounds, asbestos (1332214), azathioprine (446866), benzene (71432), benzidine (5468757), N,N-bis(2-chloroethyl)-2-naphthylamine (494031), bis(chloromethyl)ether (542881), 1,4-butanediol-dimethylsulfonate (55981), certain combined chemotherapy for lymphomas, chlorambucil (305033) chromium (7440473) and certain chromium compounds, coke over emissions, conjugated estrogens, cyclophosphamide (50180), diethylstibestrol (56531), melphalan (148823), methoxsalen (298817) with ultraviolet-A therapy, mustard gas (505602), 2-naphthylamine (91598), nickel (7440020) refining, thorium-dioxide (1314201), and vinyl-chloride (75014). The book also contains an index of synonyms and trade names.