1. Triphenyltin acetate (TPTA) has been shown to exert in vivo a selective toxic effect on the immune system. To assess in vitro possible alterations induced by TPTA exposure, primary cultures of mouse thymocytes were incubated up to 24 h with graded amounts (1-12 microM) of the organotin. 2. The cytotoxic activity has been evaluated with the MTT colorimetric assay, the neutral red (NR) assay and the lactic dehydrogenase (LDH) cellular release. Cell pellets were fixed with 2.5% glutaraldehyde, resin-embedded and ultrathin sections were observed through transmission electron microscopy. 3. After 2 h of incubation, dose-dependent increases of cytotoxicity were observed in thymocytes submitted to MTT and NR tests (up to 41.43% and 18.9%, respectively), while 22 h later this overt effect on cell viability was noticed merely in cells exposed to 12 microM TPTA. Dose-dependent increases of LDH leakage in the culture medium were observed all throughout the study. 4. Morphological investigations revealed features (chromatin condensation, cell membranes fragmentation and formation of membrane bound apoptotic bodies) suggestive of apoptosis. 5. This study indicates that TPTA is cytotoxic to mouse thymocytes: morphologically, the rising of apoptosis is likely to be recognized, as previously reported in different in vitro studies with other immunosuppressive agents as dioxin and corticosteroids.