Male and female Theiller's Original strain mice were fed for 16 and 17 months respectively on diets in which cake, prepared from flours treated with 0, 1250 or 2500 ppm chlorine, formed 79% by weight on a 12.6% moisture basis. Body weights and food intakes were unaffected by flour treatment but all of the animals on cake diets showed significant increases in body weight compared with controls on a standard diet and became obese. Mortalities in the males were not related to treatment, but in the females there was excess mortality in the treated groups compared with the cake control group, after 13 months in the 1250 group and after 15 months in the 2500 group. No consistent treatment-related effects were observed in the haematological, biochemical and renal-function studies. Dose-related increases in heart and kidney weights and a dose-related decrease in ovary weight were seen in females. No evidence of carcinogenicity resulting from flour treatment was obtained but the early ending of the study, necessitated by high mortalities, greatly diminished the value of this finding. Concentrations of covalently bound chlorine in the perirenal fat were positively correlated with treatment level, but were considerably below those present in the lipid content of the diets on which the mice were fed.