Injection of concentrated EBV derived from cells of the Kaplan line of infectious mononucleosis (IM) origin resulted in malignant lymphoproliferation in one out of three cotton-top marmosets 6 weeks after inoculation. Two additional animals receiving the same isolate after incubation with an antibody-containing human serum did not develop tumors. Inoculation of concentrated virus derived from the P3HR-1 line of Burkitt origin did not lead to lymphoproliferations in five marmosets. Three of these received non-neutralized, and two received neutralized P3HR-1 virus. The tumor obtained with the Kaplan isolate revealed characteristics of a lymphosarcome. It contained EBV-specific DNA. In addition, EBV-synthesizing lymphoblastoid lines were established from a tumorous lymph-node, as well as from the spleen of the diseased marmoset. Virus recovered from these lines transformed lymphocytes derived from spleens of healthy marmosets. The tumor-bearing animal developed low levels of anti-VCA antibodies during the course of tumor growth. These data demonstrate the oncogenic potential of EBV directly derived from cells of IM origin.