Sera from inlfectious mononucleosis patients aged 2 to 35 years and from normal subjects aged I I to 35 years were examinedfor the presence of antibody to the EB virus capsid antigen ( V C A ) Extracts of Burkitt lymphoma and other lymphoblastoid cell lines contain a soluble (S)antigen demonstrated by complement-fixation (CF) test (Armstrong P I ul., 1966;Vonka et al., 1969;1970a; 1970b;Pope et al., 1969;Gerber and Diehl, 1970). This antigen was found in both EB-virus-positive and -negative cell lines and was immunologically distinct from the viral capsid antigen (VCA) as detected by immunofluorescence (IF) or CF test. The S antibody, although present in many human sera, was never found in sera which were free of antibody to VCA. This indicates that EB virus infection is the pre-requisite for the S antibody development. On the basis of these observations the suggestion was made that the S antigen is an EB virus coded non-structural antigen.When studying the age distribution of VCA and S antibodies in human sera we found that 50 to 70% of subjects over 1 year of age, whose sera were VCA-positive, also possessed the S antibody. The antibody pattern was markedly different in children aged 6 to 12 months: only one out of 15 VCA-positive subjects in one group (Vonka et al., 1970a) and none of 30 VCApositive in the other group (Vonka and BenyeshMelnick, 1970) possessed the S antibody. The absence of S antibody in VCA-positive subjects aged 6 to 12 months, i.e. subjects most probably infected with EB virus only recently,