Lymphoid organs of male and female mice of 4 strains (ICR: CD-1, BALB/c, C57BL/6 and DBA/2) were histologically and biochemically examined at 24 hours after oral inoculation of T-2 toxin (0, 2.5, 5 and 10 mg/kg b.w.). Light microscopically, dose-dependent decrease in number of lymphocytes was observed in the thymic cortex and splenic follicles. The nuclei of lymphocytes showed pyknosis or karyorrhexis, and they were positively stained by the modified TUNEL method which detects fragmented DNA in situ. Electron microscopic characteristics of damaged lymphocytes were shrinkage of the cell body, nuclear chromatin condensation and fragmentation. Agarose gel electrophoresis of DNA extracted from the thymus showed DNA fragmentation into nucleosome units, i.e. ladder formation. The above-mentioned findings clearly showed that T-2 toxin could induce apoptotic cell death in the lymphoid organs of mice. These changes were more prominent in female BALB/c and C57BL/6 mice.