Female ICR:CD-1 mice orally treated with 10 mg/kg b.w. of T-2 toxin were killed at 1, 3, 6, 9, 12, 24 and 48 hr after treatment (HAT) and subjected to examination of the process of the development of T-2 toxin-induced apoptosis in the thymus and spleen. The early ultrastructural changes in lymphocytes characterized by shrinkage of the cell body and condensation of nuclear chromatin were detected at 3HAT in the thymus. The number of apoptotic lymphocytes observed by the in situ detection method for fragmented DNA increased drastically from 9 to 24 HAT in the thymus while it began to increase at 12 HAT in the spleen. The DNA ladder was first detected by agarose gel electrophoresis at 9 HAT and became clearer at 12 and 24 HAT in the thymus but was not clearly detected in the spleen throughout the observation period. Thus T-2 toxin-induced apoptosis developed earlier and was apparently severer in the thymus than in the spleen. Apoptotic was first detected by electron microscopy, then by the in situ detection method for fragmented DNA, and finally by DNA agarose gel electrophoresis.