Three strains of Epstein-Barr virus (EBV), two from Burkitt lymphoma (BL) and one from infectious mononucleosis (IM) were used to transform separate cultures of the same batch of primary marmoset leukocytes, and the viruses released from the transformants were compared. The three viruses shared properties of the transforming biotype of EBV, namely, stimulation of DNA synthesis and immortalization of cord blood leukocytes, and failure to induce "early antigen" in lymphoblast lines. All viruses produced more virus in transformed marmoset cells than in transformed human cells, as measured by the number of EBV genomes detected by complementary RNA/DNA hybridization, by virus capsid antigen expression, or by released virions and biologically active virus. Reference human sera and sera from primary EBV infections were used to compare the three virus strains in a virus neutralization test based on inhibition of stimulation of DNA synthesis. Specimens taken late in convalescence from patients with mononucleosis and sera from marmosets experimentally infected with virus from a patient with mononucleosis neutralized the homologous virus, as well as the two virus strains isolated from patients with BL. This finding indicates that viral antigens that elicit neutralizing antibodies are shared among the strains. However, in certain sera the neutralizing-antibody titer against one strain was consistently higher than against another strain. Furthermore, sera taken early after onset of IM contained low levels of neutralizing antibody against IM-derived virus, but failed to neutralize BL-derived virus. These latter findings suggest the existence of heterogeneity among surface antigens of EBVs. The results emphasize the biological and antigenic similarity of EBV isolates from BL and IM and do not suggest major subtype variations. It remains to be determined whether antigenic diversity such as described or virus genome variation detectable by other means is epidemiologically significant.