Previous studies in the rat have shown that bis(tri-n-butyltin) oxide (TBTO), used as a biocide, was immunotoxic at dose levels that did not affect other organs. In order to determine a no-effect level, weanling rats were treated for at least 28 consecutive days with TBTO at 0, 0.5, 2, 5, or 50 mg/kg of diet. Studies on clinical chemistry, hematology, pathology, and immune function, that is, plaque-forming cell (PFC) assay, delayed-type hypersensitivity (DTH) response, and the splenic clearance of Listeria monocytogenes, were performed at the end of treatment. No treatment-related effects were noted on clinical chemistry and hematology parameters and on PFC and DTH response, whereas thymic atrophy and impaired clearance of L. monocytogenes were noted only at a dietary concentration of 50 mg/kg. These results confirm the thymus as a target organ of TBTO immunotoxicity. Under the conditions of these experiments the dietary concentration of 5 mg/kg, equivalent to a dose of 0.5 mg/kg body weight, represents a no observed effect level (NOEL) for immunotoxicity in the Sprague-Dawley rat.