In recent studies of cycloheximide (CHX)-induced apoptosis in sublines of established Burkitt's lymphoma cell lines (BJA-B) both with and without Epstein-Barr virus (EBV) infection, we noticed two distinct types of apoptosis morphology. In the present paper, we have classified these, and further carried out a statistical analysis of their incidence in untreated and CHX-treated EBV-free (EBV(-)) and EBV-infected (EBV(+)) BJA-B cells. METHODS Both types of apoptosis morphology demonstrated typical nuclear and cytoplasmic condensation. However, "Type 1 apoptotic cells" (AP1) maintained a spherical or ovoid shape, but "Type 2 apoptotic cells" (AP2) were typified by the lobulation of their nuclear and cytoplasmic structures to form "clover leaf" shapes. Statistical analysis of incidence: The numbers of AP1 and AP2 cells were analysed using a chi 2 test, with results as follows: EBV(-) cells underwent AP1 in preference to AP2 (90.5% versus 9.5%) (p < 0.001), whilst EBV(+) cells had comparably more AP2, making AP1 and AP2 approximately equal (49.3% versus 50.7%) (p > 0.1). In EBV(-) cells, treatment with CHX had little effect on the ratios of differing apoptotic morphology. In contrast, in the EBV(+) cells, cell death was altered from AP2 (50.7%-->25.2%) towards AP1(49.3%-->74.8%) (p < 0.001). We propose that cellular proteins known to be associated with EBV infection not only protect the cells from apoptosis, but also affect the phenotype of apoptosis. This knowledge may be useful for defining possible mechanisms of apoptosis induction and/or inhibition in specific models.