Four herpesviruses were previously isolated from four outbreaks of lethal disease in owl monkeys. All four isolates have been shown to be antigenically closely related to each other and to Herpesvirus saimiri 1 (HVS-1) by kinetic neutralizations. The owl monkey strains also share similarities to HVS-1 and to each other with respect to host range, growth cycles and molecular weights of peptides and of DNA fragments generated by restriction endonuclease (R.E.) digestion. R.E. analysis, however, can differentiate strains by the use of certain enzymes. All four isolates share a common G-C ratio percentage with HVS-1 of 67 per cent. On the basis of these findings, we believe that these owl monkey virus isolates are strains of HVS-1.