Young male mice were given drinking water containing 50 ppm cadmium (Cd) for 3 weeks, and were killed 0, 3 and 6 weeks after the cessation of treatment. At 0 weeks, suppression in the number of splenic plaque-forming cells in response to sheep red blood cell immunization was noted 5 days after antigen injection, but not 7 days after injection. Plasma IgG concentration and thymic factor activity were unaffected at 0 weeks. The number of circulating lymphocytes tended to be less in the Cd-treated mice at all times. Cd treatment had no effect upon liver and kidney weights, and upon the weights and the lymphocyte contents of the thymus and spleen at any of the observation times. Employing immunofluorescence with anti-mouse IgG and C3, no evidence of an autoimmune response was found in the kidney of the treated mice at 0 and 3 weeks. Mitochondrial abnormalities in the renal proximal tubule cells were noted at 0 weeks in the Cd-treated mice. The Cd concentrations of the liver and kidneys remained high at all observation times. The results suggest that a modest dose of Cd produces some depression of the immune system, and the biological half-life of Cd is long.