Activation of Epstein-Barr virus latent genes protects human B cells from death by apoptosis. Epstein-Barr virus (EBV), a human herpesvirus, establishes a persistent asymptomatic infection of the circulating B-lymphocyte pool. The mechanism of virus persistence is not understood but, given the limited lifespan of most B cells in vivo, it seems most likely that EBV-infected cells must gain access to the long-lived memory B-cell pool. Here we show in an in vitro system that EBV, through expression of the full set of eight virus-coded 'latent' proteins, can protect human B cells from programmed cell death (apoptosis), the deletion mechanism which normally restricts entry into memory. We have found that EBV-positive Burkitt's lymphoma (BL) cell clones retaining the original tumour cell phenotype and expressing only one of the virus latent proteins, the nuclear antigen EBNA 1, are extremely sensitive to apoptosis; in this respect they resemble the tumour's normal cell of origin found in the germinal centres of lymphoid tissue. By contrast, isogenic BL cell clones which have activated expression of all eight EBV latent proteins are resistant to the induction of apoptosis. The EBV latent proteins should therefore be seen not just as activators of B-cell proliferation but, perhaps more importantly, as mediators of enhanced B-cell survival. 