This article investigates the effects of di-n-octyl tin dichloride (DOTC) on some morphologic changes in spleen and thymus as well as on the immune reaction of rats. One i.v. injection of DOTC in a dose of 4 mg/kg led in female rats to an atrophy which was reversible within six weeks, and to cell depletion of the thymus. The time needed for the regeneration (the thymus indicated a dose-dependency of the action of DOTC (4-12 mg/kg) on these morphologic changes, which was established by examining immunostimulated rats. Changes in the spleen were observed only with extremely high doses of DOTC. The action of DOTC after one or repeated immunization of rats erythrocytes taken from sheep, on the daughter-cell-dependent production of antibodies was examined by means of an immuno-rosette-test and by determining the hemagglutination titre. The number of lymphocytes in the spleen which form rosettes was reduced after one intravenous administration (4 mg/kg) of DOTC 1, 2, 3, or 8 d prior to antigenic stimulation. The time needed to form rosettes was dependent on the dose. The immunoglobulin-G-immunoglobulin-M antibody titre remained unaffected if DOTC was given once by i.v. injection 7 or 8 d prior to immunostimulation, the doses ranging from 4 to 12 mg/kg. DOTC-treatment carried out twice with 4 mg/kg led to a complete suppression of the immunoglobulin-G/immunoglobulin-M antibody production against erythrocytes taken from sheep, if DOTC had been applied on eight consecutive d and on d 0 (the d of the primary immunization).(ABSTRACT TRUNCATED AT 250 WORDS)