The recent discovery of an Epstein-Barr virus (EBV)-related lymphocryptovirus (LCV) naturally infecting common marmosets demonstrated that gamma-1 herpesviruses are not limited to human and Old World nonhuman primate hosts. We developed serologic assays to detect serum antibodies against lytic-and latentinfection marmoset LCV antigens in order to perform the first seroepidemiologic study of LCV infection in New World primates. In three different domestic colonies and in animals recently captured from the wild, we found that the seroprevalence of marmoset LCV infection was not as ubiquitous as with EBV or Old World LCV. These biologic differences in LCV infection of New World versus human and Old World primate hosts correlate with the evolution of the LCV viral gene repertoire.Serologic responses to lytic and latent infection antigens, such as viral capsid antigen (VCA) and EBNA-1, are widely used to document Epstein-Barr virus (EBV) infection (8, 13). Old World (4), and more recently New World (2), nonhuman primates are known to be naturally infected with related herpesviruses in the same lymphocryptovirus (LCV) genus as EBV. LCV infection in Old World primates was initially recognized by the presence of serum antibodies cross-reactive with viral antigens in EBV-infected B cells (7). As with humans, LCV seropositivity in Old World primates is highly prevalent both in nature and in domesticated colonies, with seropositivity in more than 95% of adult animals (5, 7, 9). The biology of LCV infection in Old World primates appears to be nearly identical to that of EBV infection in humans (16). This is concordant with the identical repertoire of viral genes and the high degree of sequence homology between EBV and rhesus LCV, a prototype for an Old World LCV whose genome has recently been fully sequenced (11).It was long believed that LCV did not infect New World primates, since there was no strong evidence of EBV crossreactive antibodies from these species. However, we recently isolated a B-cell-immortalizing herpesvirus from a spontaneous B-cell lymphoma arising in a common marmoset (Callithrix jacchus) that belonged to the LCV genus (2). PCR studies showed that LCV infection was present in three different marmoset colonies, and a similar LCV was also found in squirrel monkeys, indicating that LCV infection is present in species from both major families of New World primates (2). Studies from other laboratories have since confirmed that LCV can be found in a variety of New World species (3, 6).The complete genome sequence of the marmoset LCV is striking for its differences from the EBV genome (10). The marmoset LCV encodes at least seven unique open reading frames (ORFs) with no sequence homology to other cell or viral genes. In most cases, there is compelling evidence that these unique genes are predecessors for certain EBV genes, e.g., they are located in the same position of the genome, they have similar transcriptional patterns, they have similar predicted secondary protein structures, or they have similar functio...