In order to characterise the mechanism of cytotoxicity of the immunotoxic organotin compound bis(tri-n-butyltin)oxide (TBTO) to lymphoid cells, isolated thymocytes from immature rats were exposed to TBTO (0.1-5 microM) for up to 6 h. At lower TBTO concentrations (0.1 and 1 microM) vital staining showed that only marginal loss of viability occurred, although morphological studies demonstrated increased numbers of cells with abnormal features indicative of programmed cell death (apoptosis). These changes included nuclear chromatin condensation (which was associated with increased DNA fragmentation), cytoplasmic contraction and formation of membrane bound apoptotic bodies. When visualised by agarose gel electrophoresis, genomic DNA appeared as a series of fragments with a repeat multiple of 180-200 base pairs. Comparable morphological changes and cleavage of DNA into oligonucleosomal fragments were evident in thymocytes incubated with 10 microM methyl prednisolone hemisuccinate (MPS); a glucocorticoid hormone known to induce programmed cell death in thymocytes. Marked cytotoxicity associated with degenerative changes indicative of necrosis was observed in thymocytes incubated with 5 microM TBTO. These findings indicate that, at levels which are not overtly cytotoxic, TBTO is capable of inducing programmed cell death in rat thymocytes. This suggests a possible mechanism for the T-cell immunodeficiency previously reported for TBTO in vivo.