Six cotton-topped marmoset monkeys (Sangiunus oedipus) were inoculated with 10(5) transforming units of B95-8 virus, and two of them developed fatal lymphoproliferative disease. The EBV-carrying tumor cells from these marmosets had the following characteristics: (1) they were polyclonal by surface immunoglobulin and immunoglobulin production in vitro; (2) they had no specific chromosome abnormalities, and (3) they failed to form colonies in large percentages in agarose. It is proposed that a spectrum of phenotypes of EBV-induced lymphoproliferative diseases in the cotton-topped marmosets may be identified and are more akin to fatal infectious mononucleosis or X-linked lymphoproliferative syndrome than to Burkitt's lymphoma.