On February 6, 1978, two maintenance workers employed by a chemical company in Atlanta, Georgia, became ill after cleaning a clogged drain. Both were hospitalized with acute fulminant hemolytic anemia and renal failure. While the clinical picture suggested arsine or stibine poisoning, preliminary investigation of the plant revealed no obvious source of arsenic, antimony, or hydrogen gas. During the cleaning operation, the men drained a mixing tank that 5 yr before had been used to store arsenical herbicides. To unclog the drain, they added a standard drain cleaner containing sodium hydroxide and aluminum chips, a combination that reacted to release hydrogen gas. This gas combined with the arsenic residue to form toxic quantities of arsine gas (arsenic trihydride). Arsenic was found in the liquid and gas from the drain and in blood and urine of both patients, and of two other workers who worked in the vicinity of the drain. This investigation suggests that drain cleaners which react to release hydrogen should not be used in situations where arsenic or antimony may be present. Furthermore, maintenance men, who may be exposed to the chemicals used in a plant, should be educated, supervised, and screened so that their risk for occupational hazards may be reduced.