Fifty-nine unselected pregnant women were included in the study at the time of delivery. Immediately post partum maternal blood was obtained by venous puncture, and fetal blood from the umbilical cord. Amniotic fluid was obtained by amniocentesis during labor prior to rupture of the amniotic sac. Virus-specific rubella, herpes simplex and cytomegalo virus antibodies were determined with a commercial enzyme immune test. The virus-specific antibody titers are higher by a factor of 1.3 to 1.5 in the fetal than in the maternal serum. In contrast, the concentration of the corresponding antibodies in the amniotic fluid is approximately 5 titer levels (log 2) lower than in the maternal serum - regardless of the maternal antibody titer level. In view of its low concentration of antibodies the amniotic fluid can hardly be regarded as an "immunological barrier" if it serves as a "transport or distribution medium" in some virus infections. However, passive intra-amniotic immunization could be useful for combating certain virus infections during pregnancy, especially just before term.