Six immature infants were given oral feedings of 10% preservative-free human immune serum globulin ranging from 1 to 8 ml/kg/day. A seventh infant served as a control. Undigested and partially digested IgG was detected in the stools in significant quantities in all but the control infant. This coproantibody retained significant opsonic activity for type III group B streptococci as determined by a chemiluminescence assay, but lost most of its tetanus antibody activity. The newborn infants' enzymatic immaturity or rapid transit time permits the passage of intact IgG or partially digested IgG to pass throughout the gastrointestinal tract.