Two hundred children and their mothers were examined when they were 10 months, 2 years and 4 years of age. Anti-cytomegalovirus, anti-Herpesvirus hominis and anti-rubella virus antibodies were titered. The evolution of antibody titers showed that primary infections with cytomegalovirus and Herpesvirus are especially frequent during the first months of life, and that mothers are the principal source of infection. In the group studied, it appeared that country of origin of the parents was the most important socio-economic factor influencing the frequency of infections in children, based upon the large variations of the serum positivity in mothers.