Chicken peripheral blood lymphocytes (PBLs) show morphological and biochemical features of apoptosis (programmed cell death) when infected in vitro with infectious bursal disease virus (IBDV). DNA extracted from IBDV-infected lymphocytes displayed an intense laddering pattern when visualized after agarose gel electrophoresis. IBDV-infected PBLs had significantly higher apoptotic and necrotic indices measured by acridine orange-ethidium bromide staining than did control lymphocytes. Electron micrographs of the IBDV-infected PBLs revealed features typical of apoptosis such as peripheral condensation of chromatin, blebbing of the plasma membrane and fragmentation of the nucleus and of the cell leading to the formation of apoptotic bodies. These findings indicate that IBDV, in addition to causing necrosis, can also induce apoptosis in avian lymphocytes in vitro.