The effects of di-n-octyltin dichloride (DOTC) on the thymus in mice were studied in dependence on the route of administration. Single administrations of 30, 60 and 120 mg/kg of DOTC i.m. induced a dose-related decrease of the thymus weight and of the number of nucleated cells in the thymus of male mice. 4 days after intravenous injection of 8 mg/kg of DOTC the thymus weight and the number of thymocytes were reduced to 50 percent of the control. In oral route of administration higher doses of DOTC (300 mg/kg) were necessary to provoke a thymus atrophy. After pretreatment of mice with carbon tetrachloride the toxic effects of orally administered DOTC were increased. The reduction of the thymus weight and of the number of thymocytes in mice after parenteral administration was accompanied by a depression of thymus-dependent antibody formation against sheep red blood cells.