Paired maternal and cord sera from 100 pregnancies were tested for antibodies against herpes simplex virus, measles virus and respiratory syncytial virus by complement fixation and for antibodies against rubella virus, influenza A virus and influenza B virus by haemagglutination-inhibition. For four viruses (herpes simplex, measles, respiratory syncytial and rubella) higher levels of antibody were found in cord than in maternal sera. There was no difference between maternal and cord serum titres against influenza B virus but significantly higher levels of antibody against influenza A virus were found in maternal sera than in cord sera. This discrepancy was investigated by measuring antibodies against the surface antigens of influenza A by a complement fixation technique, and by single radial haemolysis. Both methods showed a preponderance of virus-specific antibody in cord sera. We conclude that IgG antibodies against most, if not all, viruses are concentrated on the fetal side of the circulation, but that conventional haemagglutination-inhibition techniques may fail to detect this difference.