The National Institute for Occupational Safety and Health (NIOSH) recommends that it would be prudent to handle 1,2-dichloroethane (ethylene dichloride); 1,1,2- trichloroethane; 1,1,2,2-tetrachloroethane; and hexachloroethane in the workplace as if they were human carcinogens. This recommendation is based primarily on consideration of National Cancer Institute (NCI) data indicating that laboratory animals administered these compounds experienced a statistically significant excess of cancer as compared to control animals.1-4 Additionally, NIOSH recommends that five other chloroethane compounds: chloroethane* (ethyl chloride); 1,1-dichloroethane; 1,1,1-trichloroethane (methyl chloroform); 1,1,1,2-tetrachloroethane; and pentachloroethane be closely monitored for carcinogenic effects in humans and/or laboratory animals. These five should also be treated in the workplace with caution because of their relation to the four chloroethanes shown to be carcinogenic in laboratory animals. This Current Intelligence Bulletin summarizes information on some of the similarities and dissimilarities within the chloroethane group. NIOSH is concerned about the carcinogenic potential of chloroethanes based on emerging data from the NCI bioassay program. Concern for the carcinogenic potential of all members of the chloroethane series is based upon structural similarities within the group as well as the structural similarities to other carcinogenic organochlorine compounds. Extreme care must be used when selecting possible substitutes, and the alternatives should be fully evaluated with regard to human effects. At present, NIOSH is not aware of any evidence associating chloroethane compounds with an increased risk of cancer in man. However, animal studies are valuable in helping identify human carcinogens. Substances that cause cancer in experimental animals must be considered a potential cancer risk in man. Safe levels of exposure to carcinogens have not been demonstrated, but lowered exposure to carcinogens decreases the probability of cancer development. NIOSH issued Current Intelligence Bulletin #25 (April 1978), recommending that ethylene dichloride (1,2-dichloroethane) be handled in the workplace as if it were a human carcinogen.5 NIOSH is now distributing Bulletin #27 to advise of additional findings of the NCI chloroethane bioassays, other pertinent data, and possible implications for occupational health. Also included are "Suggested Guidelines for Controlling Employee Exposure to Chloroethanes." NIOSH requests that producers, distributors, professional associations, and unions transmit the information in this Bulletin to their customers, employees, associates and members.