SummaryAn IgG specific enzyme linked inmmunosorbent assay and a complement fixation test were used in parallel to detect cytomegalovirus antibody in 1564 individuals, 1 to 95 years old, living in Belgium. With the first test, about 50 % of antibody carriers were found, but many serum specimens reacting in the enzymatic assay were negative in the complement fixation test. This was the case in 24.3 % of the male blood donors, 57.4 % of the patients with respiratory or flu-like disease and 83.7 % of the patients in acute episode of their chronic bronchitis. Most specimens from the rest of the healthy antibody carriersi.e.women after delivery, medical personnel and female blood donors, reacted in both tests. Specimens from 0.64 % complement fixation positives proved enzymatic assay negative.The results of the enzymatic assays were used to study the age-related acquisition of cytomegalovirus antibody. In order to detect differences according to the geographic origin, the investigated population was divided from the patronymics into belgians and non-belgians. More than 90 % of the latter group were mediterranean, mostly italian immigrants. There were about 45 % antibody carriers in the whole investigated belgian subpopulation and 70 % in the non-belgian. Graphic analysis of the results shows that in both subpopulations there is an increase of antibody carriers as the investigated sample includes older and older individuals. In both cases, when the age-group cumulative numbers of antibody carriers are plotted against the age-group cumulative number of investigated individuals, this increase follows a parabola, indicating a constant age-related acceleration of antibody acquisition. This acceleration is 6.6 times as high in the nonbelgian subpopulation as in the belgian. It is suggested that the high acceleration in the non-belgian subpopulation is related to a high proportion of cytomegalovirus excreting mothers and that both curves can be extrapolated to zero point, indicating that infants would be very rarely infected before birth.