Sequential morphologic changes in the lymphoid organs were examined after ocular and cloacal inoculation in 3weekold chicks with a highly virulent strain (SH/92) of infectious bursal disease virus (IBDV). The infected chickens were sacrificed at 6, 12, 24, 48, 72, and 96 hrs post inoculation (Pl), and thymus, harderian gland, ceacal tonsil, and spleen were observed. Histologically, the significant lesions were characterized by lymphocyte depletion and the earliest detectable changes were evident at 12 hrs Pl. In thymic cortex, lymphoid depletion with apoptosis and prominent "tingible body macrophages" were observed. As the infection advanced, the lesions showed more severe changes. Dying cells were characterized either by capping of nuclear chromatin (apoptosis) or by cytoplasmic swelling (necrosis). In situ staining for apoptosis, some lymphoid cells revealed typical positive reaction, even the stainability was variable depend on every lymphoid organs. These results suggest that IBBV (SH/92) cause severe damage both primary and secondary lymphoid organs, and both T and B lymphocytes. Also the lymphoid depletion of these organs is caused by necrosis and apoptosis induced by IBDV.d by IBDV.