Neutralization tests used in one laboratory in the USA and one laboratory in England to detect antibodies to Herpesvirus simiae have been compared. Complete concordance in results was obtained with 53 (90%) of 59 monkey sera. The remaining six sera all had titers no greater than 1:3. Four were positive only in the American test, and two were positive only in the British test. The importance of using complement if maximum sensitivity is to be achieved in detecting antibodies to this virus has been confirmed.