Herpesvirus ateles, nonpathogenic in its natural host, the spider monkey, induces a fatal lymphoproliferative syndrome in a variety of New World primate species. Whereas the closely related New World primate virus Herpesvirus saimiri immortalizes in vitro common marmoset lymphocytes that express a TCR and phenotypically are CD4-CD8+NKH1+, we now show that H. ateles-immortalized marmoset lymphocytes are CD3+ and CD4+. Furthermore, these CD4+ lymphocytes coexpress CD8 and NKH1. The NK function of cloned H. ateles-immortalized lymphocyte populations is proportional to the extent to which they express the CD8 antigen. These studies illustrate an interesting example of restricted viral tropism and may indicate a potentially useful means of generating phenotypically stable, functionally competent, cloned lymphocyte populations for study.