The presence of an antibody to the Cyno-EBV was studied with Cyno-EBV bearing cell line (Ts-B-9) as antigen in 186 domestically bred cynomolgus monkeys (Macaca fascicularis) ranging in age from birth to 18 yr as well as in 20 adult (older than 10 yr) cynomolgus monkeys of wild origin. The maternal antibody to Cyno-EBV persisted for less than four mo after birth in infant monkeys. After one yr of age, all the domestically bred monkeys were consistently positive for Cyno-EBV. In the monkeys of wild origin also, the anti-Cyno-EBV antibody was detected without exception. Antibody titers in adult monkeys varied from 1:80 to 1:640.