The leukocyte migration inhibition (LMI) technique was used to measure the T cell-mediated immune response of Epstein-Barr-virus (EBV)-seropositive human donors to antigens associated with B cell lines of simian origin, transformed by simian EBV-like viruses, Herpesvirus papio (HVP), H. pan, H. gorilla and H. pongo. Extracts of cell lines carrying three of the four simian viruses (from gorilla, chimpanzee and orangutan) induced a positive LMI response, whereas lines carrying baboon-derived HVP were ineffective. None of the simian virus-transformed lines elicited an LMI reaction in human EBV-seronegative individuals. Leukocytes from patients with acute infectious mononucleosis (IM) failed to respond to any of the lines transformed by EBV or the simian EBV-like viruses. Such lines express the virally encoded nuclear antigen, but have only a low level of viral cycle-associated antigens. Extracts of the EBV-carrying human cell line P3HR-1 induced with 12-O-tetradecanoylphorbol-13-acetate to express high levels of early and virus capsid antigens (EA, VCA, respectively) however, elicited a strong response with leukocytes from patients with acute IM. During convalescence, IM patients became responsive to EBV, H. gorilla-, H. pan- and H. pongo-transformed lines, indicating that the LMI reaction induced by these simian virus-transformed lines was directed against the antigens expressed in immortalized cells rather than against antigens of the lytic cycle. It is highly probable that this reaction reflects a cross-recognition of the nuclear antigens associated with these four transforming viruses (excluding H. papio) at the level of human T cells.