The toxic hazards of bis(chloromethyl)ether (542881) (BCME) are discussed. The findings of a variety of studies on BCME carcinogenicity in laboratory animals are summarized, including skin painting tests in Swiss-mice, injection tests in rats, subcutaneous injection tests in Swiss-mice, inhalation exposure tests in mice and Sprague-Dawley-rats. Occurrences of fibrosarcomas, fibromas, and lung adenomas in these animals are discussed. The results of a NIOSH survey of bronchial damage and lung cancer incidence among workers exposed to BCME at an anion exchange resin production factory are reviewed, including an increased incidence of abnormal sputum cytology and of lung cancer, particularly undifferentiated oat cell carcinoma. An increased incidence of lung cancer among workers exposed to chloromethyl-methyl-ether (3188134) is discussed with respect to the probable concomitant exposure to BCME. Lung cancer deaths are also discussed among 8 of 68 workers from a facility where BCME exposures occurred. The formation of BCME from the reaction of hydrogen-chloride with formaldehyde is described. The author concludes that BCME may be one of the most potentially hazardous carcinogens found in the workplace. The possibility of BCME formation must be considered wherever both hydrogen-chloride and formaldehyde are present. The most stringent measures should be used to prevent any worker exposures to BCME.