Sera from 721 randomly selected infants were examined for antibodies to Epstein-Barr virus (VCA and EA) and the prevalence by age was determined.1) Antibodies to VCA were found in 82% of 0-month old infants but decreased rapidly to only 7.4% (3.7-19.2%) in infants aged 3-9 months. They then increased slowly with age and reached a plateau (80-90%) by the age of 8-9 years. 2) Antibodies to EA were found in 15 (4.3%) out of 347 EBV-VCA seropostive infants. All were R type.3) The percentage of children aged from 3 months to 8 years having antibodies decreased apparently as compared to the results of Sakai which were obtained about 7 years previously. These results reveal that the prevalence of EBV is now decreasing to the level in White Americans and also suggest that clinical cases of infectious mononucleosis may be expected to increase in Japan.(Abstract of Nippon Shonikagakhi Zasshi (Acta. Paed. Jpn. Japanese ed.) 86(11).1982) Acta Paediatr Jpn