A carcinogen bioassay of enflurane was performed in Swiss/ICR mice. Two groups of weanling mice, each of 125 males and 125 females, were exposed to either a maximum tolerated dose of enflurane, 3000 ppm (0.3 per cent v/v) or compressed air for 4 hs per day, 5 days per week. After 52 weeks exposure, 25 males and 25 females from each group were killed. After 78 weeks exposure, a 4-week period without treatment was allowed before the remaining mice were killed. Mice killed at the scheduled periods and those killed or dying at other times throughout the study, underwent extensive gross and histologic examinations unless autolysis or cannibalism precluded examination. After 78 weeks exposure, male mice in the enflurane-treated group had a 36 per cent incidence of liver tumors compared with 24 per cent in the control group; however, the difference was not statistically significant. The incidence of lung tumors was about 20 per cent in all groups. Other neoplastic lesions occurred in small numbers and were unrelated to treatment. It was concluded that under the conditions of the present experiment, chronic administration of enflurane at its maximum tolerated dose did not lead to an increased incidence of neoplasia in Swiss/ICR mice.