EB virus is a herpesvirus that infects all human communities. The infection is life-long and usually asymptomatic. Excessive reaction to primary infection leads to infectious mononucleosis while immunological failures give fatal lymphoproliferative diseases. The virus is associated with endemic Burkitt's lymphoma and undifferentiated nasopharyngeal carcinoma. In world cancer terms Burkitt's lymphoma is insignificant, but nasopharyngeal carcinoma has a high incidence in certain important populations. By analogy with herpesvirus-induced animal cancers, prevention of infection should greatly reduce subsequent development of tumours. A prototype vaccine has therefore been produced based on the virus-determined antigen (MA gp340) that elicits virus-neutralizing antibodies. A sensitive assay has permitted the elaboration of an efficient antigen preparation method and the product has been rendered highly immunogenic, as tested in mice and rabbits, by incorporation in liposomes. The only animal suitable for experimental EB virus infection is the little-known cottontop tamarin; a breeding colony has been successfully established and protection against virus challenge assessed in immunized tamarins. The overall structure of the antigen has been determined in preparation for future production by synthesis or rDNA technology.