The New World primate species Sanguinus oedipus, the cotton-top tamarin, has a high incidence of spontaneously occurring adenocarcinoma of the colon and develops a fatal lymphoproliferative syndrome following infection with various herpes viruses. Some investigators have linked such disease susceptibilities to abnormalities in the immune function of the cotton-top tamarin that may result from the natural bone marrow chimerism that occurs in this species. The present studies were initiated to establish conditions for studying the immune system of these primates and to assess the integrity of this system. First, we document the state of bone marrow chimerism of S oedipus. We then show that standard in vitro assays of lymphocyte function can be done with cells from this animal. Proliferative T cell assays, cytotoxic T lymphocyte (CTL) assays, and measurements of CTL precursor frequency all indicate that the immune system of this animal functions normally. Finally, we demonstrate in vitro the tolerance of lymphocytes from one S oedipus for the cells of its twin. The ability to manipulate tamarin blood cells in vitro will allow future investigation into mechanisms of immune tolerance and major histocompatibility complex restrictions on cell cooperation in this species.