Summary Radioactively labeled Herpesvirus ateles (HA) was partially purified by sedimentation in sucrose. The virus banded in a potassium tartrate gradient at a density of 1.18 g/ml. The DNA extracted from the partially purified virus cosedimented in a sharp peak with herpes simplex virus type-2 DNA in a neutral sucrose gradient. The sedimentation constant of HA DNA was calculated at 58±2 s20.w, with T4 phage DNA as a marker. The partially purified virus induced malignant lymphoma in cotton-top (CT) marmosets (Saguinus oedipus). The virus was isolated from the lesions by cocultivation of the tumor cells with owl-monkey kidney (OMK) cells. Lymphoid cells derived from a tumorous lymph node synthesized infectious HA in vitro. The virus genome was transmitted by transplantation of tumor cells from a CT marmoset into 2 Callithrix jacchus marmosets. The transplantation of tumor cells resulted in a carrier state of HA in the peripheral blood cells of the C. jacchus marmosets for 207 days. The injection of cell-free HA induced malignant lymphoma in 2 other animals of the same species. In the sera of all new-world monkeys (S. oedipus, C. jacchus, and Aotus trivirgatus) inoculated with HA or tumor cells, neutralizing antibodies against HA were demonstrated, and infectious HA was isolated by cocultivation of fresh peripheral blood from these monkeys with OMK cells. The old-world monkeys (Cercopithecus aethiops) inoculated with HA or tumor cells did not develop malignant lymphoma, and HA could not be isolated from their peripheral blood cells.