Owl monkeys (Aotus trivirgatus) were inoculated with EB (Epstein-Barr) virus-containing material or virion-free control material. One out of three animals given the virus died after 14 weeks with a reticuloproliferative disease compatible with malignant lymphoma. A cell line was established from an abnormal lymph node of the diseased monkey and was found by electronmicroscopy to carry a herpesvirus identified as EB virus by two types of immunofluorescence test, by its biological behavior and by nucleic acid hybridization studies. None of the inoculated animals developed heterophile antibodies, and only the diseased animal had an antibody response to EB virus. The findings were discussed in relation to a possible oncogenic capacity of EB virus in vivo.