Organotins are used worldwide in agricultural practice as fungicides and herbicides. In this study morphological and ultrastructural investigations related with the subacute administration of the fungicide triphenyltin acetate (TPTA) were carried out in rabbits and lambs. Twenty-eight New Zealand White male rabbits were fed diets containing 0, 15, 75 or 150 ppm TPTA for 70 d; comparable doses (1 or 7.5 mg/kg bw) were administered daily to immature male lambs. After 70 d of treatment dose-dependent decreases in body weight gain and thymus relative weights were seen in both species. In rabbits, the main histological lesions were found in the thymus and mesenteric and retropharyngeal lymph nodes, confirming the immunosuppressive activity reported by TPTA in other rodents. Lambs showed similar, but less severe lesions. However, the involvement of the immune system was noted in both species, but at doses much higher than those reported for rats and guinea pigs. This different immunotoxic activity of TPTA might be related to species differences in the toxicokinetics of the fungicide.