A novel member of the papovavirus group has been isolated from EBV-transformed African green monkey (AGM) B-lymphoblasts. The virus is characterized by its B-lymphotropic host range and has tentatively been named "lymphotropic papovavirus" (LPV). Seroepidemiological studies revealed that besides sera from African green monkeys a substantial proportion of human sera reacted with antigens of this virus. The specificity of this reaction observed in indirect immunofluorescence tests was underlined by immunoprecipitation (IP) and neutralization studies. By IP we demonstrated that AGM as well as human sera reacting in immunofluorescence with LPV antigens precipitated polypeptides of about 40 K. Those polypeptides were detected only in LPV-infected cultures. Sera of high reactivity in immunofluorescence tests also neutralized viral infectivity. These data suggest that an agent antigenetically closely related to AGM-LPV also infects part of the human population. The age distribution of the human antibody response revealed that a low percentage of sera reacted in age groups below 30 years. Thereafter, however, about 30% of all sera exhibited antibodies to LPV antigens. There was a slight increase in seroreactivity in sera from patients tested for infectious hepatitis when compared to non-selected human sera.