Antibodies to E. coli O-antigens and poliovirus type I antigen as well as total SIgA were analysed using enzyme-linked immunosorbent assay (ELISA), in amniotic fluid and in meconium, urine and saliva from neonates. Secretory IgA and IgM antibodies to E. coli and poliovirus antigens were found in saliva as well as in most meconium samples taken during the first day of life. SIgA could be quantified in all types of samples including amniotic fluid. The finding of secretory IgA and IgM antibodies to E. coli and poliovirus type I antigens in early samples from an infant of a hypogammaglobulinaemic mother, given regular intravenous (i.v.) immunoglobulin prophylaxis, but still lacking IgA and IgM antibodies, supports a fetal origin for at least part of the secretory antibodies detected in the different samples. Since it is unlikely that the fetus has been exposed to poliovirus, which is rare in Sweden, it is hypothesized that the stimulus inducing the SIgA and IgM antibodies found in the neonate could be anti-idiotypic antibodies from the mother.