A lymphotropic papovavirus was isolated from a lymphoblastoid cell line of African green monkey (AGM) cells which also contained a herpesvirus and a paramyxovirus-like agent. The papovavirus was analyzed by restriction endonuclease cleavage; its biochemical and serological crossreactivity with SV40 and host range have been determined. Thus far, only B-lymphoblasts of primate and human origin have been found to be susceptible to infection. Although more than 50% of the tested monkey sera were reactive with antigens of this virus, all human sera tested failed to react. Cleavage patterns and hybridization studies with the viral DNA indicate that the virus represents a novel member of the papovavirus group that is characterized by its lymphotropic host range. Papovavirus particles were also demonstrated in a human lymphoblastoid cell line (CCRF-SB) originally derived from a leukemic child. These cells revealed nuclear fluorescence when tested with human sera, but failed to react with AGM sera. Although characterization of this agent has not yet been completed, available evidence suggests that it represents another lymphotropic papovavirus which seems to be spread within the human population.