Antibodies to Epstein-Barr (EB) virus capsid antigen and EB virus complement fixing antibodies were estimated in sera from 23 children and 27 young adults with infectious mononucleosis, and in 79 control patients. Complement fixing antibodies, but not those to virus capsid antigen, were much lower in patients tested during acute infectious mononucleosis than in controls. It is suggested that this disparity could form the basis of a diagnostic test. Anomalous results, in which the EB virus antibody pattern suggested active infection though heterophile antibodies and/or abnormal white blood cells were absent, were observed in sera from patients of all ages but the majority were aged 5 years or less. The proportion of sera with EB virus antibodies increased throughout childhood, suggesting that infection with this virus is relatively frequent in this age group and may often be unrecognized.