It is generally agreed that cellular immunity plays an important role in limiting certain primary viral infections. Morphological studies indicate that cell death induced by T cells, K cells and NK cells takes the form of apoptosis, not classical necrosis. Killing of a virus-infected cell by either of these means prior to the assembly of infectious virus would clearly contain the infection. Our hypothesis is that the exclusive involvement of apoptosis in lymphocytotoxicity may have additional advantages in preventing virus dissemination. Firstly, a very early event in apoptosis is activation of endogenous, non-lysosomal endonuclease, and this might destroy virus. Secondly, apoptosis results in the formation of membrane-bounded cell fragments, which are phagocytosed intact and digested within the lysosomes of adjacent cells. In contrast, necrosis is characteristically associated with rupture of the cell membrane and release of cellular contents; its induction by non-budding viruses aids in spread of the infection.