Passive transfer of immunity from the mother to the first trimester fetus is of particular interest because of the reported high incidence of serious fetal sequelae due to congenital infection. We have examined the relationship between maternal serum, coelomic fluid and amniotic fluid concentrations of immunoglobulin (Ig) and complement. Ig fractions G (IgG), A (IgA), and M (IgM) and complement factors 3 (C3) and 4 (C4) were measured in 34 normal pregnancies between 6 and 12 weeks of gestation. The concentrations of specific antibodies for Toxoplasma gondii, cytomegalovirus (CMV) and rubella were also measured on 21 matched samples from the same study group. IgG and IgA concentrations were detected in all coelomic fluid samples whereas IgG was only measurable in two amniotic fluid samples. IgG and IgA concentrations were respectively 28 and 128 times lower in coelomic fluid than in maternal serum and probably reflect fetal serum concentrations. Significant positive linear correlations were found between gestational age and the coelomic concentrations of IgG (P = 0.001) and IgA (P = 0.014). There was no obvious association between immunoglobulin concentrations in coelomic fluid and maternal serum suggesting increasing active transport across the placenta with advancing gestation. IgM, C3 and C4 were not detected in coelomic or amniotic fluid samples. Specific antibodies were found in 13 out of 63 samples of coelomic fluid and in 32 out of 63 samples of maternal serum. They were found in coelomic fluid only if they were present in maternal serum. These results suggest that maternal IgG and IgA are potentially available to the embryo as early as the 6th week of gestation. The presence in the coelomic fluid of the IgG fraction, both total and infectious agent-specific transferred via the placenta, indicates that they may provide limited protection against congenital infection in the first trimester.