This letter to the editor described some preliminary results of a study being performed for NIOSH in which male and female Sprague- Dawley-rats were exposed to ethylene-dibromide (106934) (EDB). Exposure was at the current United States Occupational Standard of 20 parts per million (ppm). Disulfiram was chosen as a potential modifier of toxicity because of its known effect as an enzyme inhibitor and its use in humans in alcoholism control programs. The four different conditions of the study were as follows: control/control, filtered air exposed, standard rat diet; control/disulfiram, filtered air exposed, diet containing 0.05% disulfiram by weight; EDB/control, 20ppm EDB, standard rat diet; EDB/disulfiram, 20ppm EDB, diet containing 0.05% disulfiram by weight. After 11 months of exposure, a substantial increase in morbidity and mortality were noted in both male and female rats in the EDB/disulfiram group. In addition, a large number of tumors occurred at various sites in both male and female rats in this group. Other potentially important findings included hemangiosarcoma of the liver, spleen, omentum, kidney, and adenocarcinoma of the mammary gland, and tubular atrophy of the testes. The data suggested the existence of an interaction between EDB and disulfiram in the rat, but the clinical significance in man was not known.