The authors have investigated the presence of serum-derived immunoglobulin G (IgG) fragments in the human intestine at various ages, these fragments possibly representing another source of antibodies in addition to secretory IgA (SIgA). Fab fragments of the gamma isotype were found to be the major molecular form of immunoglobulins in the meconium (median value: 3.7 mg/g of stools), as compared with Fab alpha (75 micrograms/g) and IgM (2.6 micrograms/g). These fragments provided by molecules of the maternal serum displayed a strong antibody activity to the tetanus toxoid and were also found in the stools of 1-week-old babies fed with formula milk. The release of serum antibodies into the digestive lumen occurs largely via hepatobiliary secretions, as suggested by the presence of IgG antitoxins in the bile of children operated on extrahepatic biliary atresia. In adults, the Fab antitoxins were also detected in most stool extracts. Affinity of these molecules was found to be similar to that of their serum counterpart with a Ka of 2.1 x 10(10) M1 (median value). These mucosal antibody fragments, associated with the normal pathway of serum IgG catabolism, could provide additional immune defences against pathogens, and be of importance to supplement an immature or deficient secretory immune system.