Neonates of some species of mammals absorb water-soluble macromolecules from the lumen of the gut to the circulation. This is a means for providing the neonate with passive immunological protection. The accepted model for absorption of macromolecules, particularly immunoglobulin G (IgG), has at least three phases: adherence of the macromolecule to the brush border on enterocytes; internalization of the macromolecule within the enterocytes; and egress of the macromolecule into the lamina propria. With regard to the absorption of IgG, adherence is thought to be a specific reaction of ligand (IgG) with a plasmalemma binding site. Pinocytosis is activated and internalization follows. Egress into the lamina propria occurs at the basal-lateral membrane by a process of reverse pinocytosis. Unbound (unprotected) macromolecules that are internalized in the pinocytosic fluid are shunted off to lysosomes and either digested or stored therein. Neonatal rodents fit this model for macromolecular absorption. However, in another group of neonates (e.g., pig, cow, horse), nonselected absorption takes place, in that IgG and other macromolecules are transported from the gut lumen to the blood. In a third group of neonates, (e.g., human, guinea pig) absorption of IgG is either of low order or nonexistent. Since neonatal mammals possess a mechanism for absorbing macromolecules, there is the potential for internalizing toxic macromolecules if the toxin is presented to the neonate's enterocytes in competitive amounts. Adults retain remnants of the neonatal absorptive mechanism.