A strong association exists between Epstein-Barr (EB) virus and two human cancers, endemic Burkitt's lymphoma and nasopharyngeal carcinoma. In addition, the virus causes infectious mononucleosis [reviewed in Epstein and Achong, 1979, 1986] and more recently has been implicated in lymphomas arising in immunosuppressed individuals [Cleary et al., 1986]. The possibility of preventing or influencing the course of these diseases by vaccination has been advocated for a number of years [Epstein, 1976], especially in the case of undifferentiated nasopharyngeal carcinoma, which is the most common tumour of men in southern China and is prevalent in other specific regions; it therefore represents a major world cancer problem [Shanmugaratnam, 1971]. Two vaccinia virus strains were employed to make recombinants expressing the gene coding for the EB virus envelope glycoprotein, gp340, and were used to vaccinate cottontop tamarins. Protection against EB-virus-induced lymphoma was obtained in animals immunized with the laboratory (WR) strain recombinant but not with those recombinants derived from the vaccine (Wyeth) strain. Circulating antibodies to EB virus gp340 were not detected in any of the immunized animals.