To perform a better investigation of the toxic activity of triphenyltin acetate (TPTA) already described in vivo, primary cultures of murine thymocytes were incubated for 2 to 32 h with graded amounts (0.5-8 microM) of the triorganotin compound. The cytotoxic activity has been evaluated with the Trypan blue dye exclusion test, the 3-(4,5-dimethyl-thiazol-2-yl)-2,5 diphenyl-tetrazolium bromide (MTT) assay for cell survival and the cellular release of lactate dehydrogenase. Following 2 h of incubation with TPTA, a dose-dependent reduction (P < 0.05) of cellular viability occurred and marked increases (P < 0.05) of the MTT cytotoxic index and of lactate dehydrogenase leakage were also observed. These findings indicate that TPTA is as cytotoxic to mice thymocytes, as other triphenyltin derivatives are to other species.