Previous work dealing with the acquisition of passive immunity by the neonatal mammal has shown that the piglet is qualitatively nonselective with regard to the transport of macromolecules from the gut to the blood, whereas rats, mice and hamsters transport only immunoglobulin G (IgG) in detectable quantities. The ability of enterocytes of the proximal and distal small intestine of piglets to internalize fluorescent porcine IgG, bovine albumin or a mixture of these two fluorescent proteins was assessed. At the light microscopic level (macropinocytosis) porcine IgG and bovine albumin were internalized with equal facility by these enterocytes. To determine whether piglets preferentially transported porcine IgG versus bovine albumin, other piglets were gavaged with these proteins or these proteins were injected into ligated segments of the proximal or distal small intestine. Enterocytes in the proximal part of the small intestine transported more of these proteins (P less than 0.10) than did those in the distal part. When either bovine albumin or porcine IgG was presented separately to the piglet's gut, both were transported to about the same low level. However, when mixtures of these proteins were presented, IgG was preferentially transported, i.e., albumin enhanced IgG transport. Porcine albumin or polyvinylpyrrolidone (PVP) also enhanced porcine IgG transport. It was proposed that the preferential transport of IgG occurred by a micropinocytotic mechanism (not visible by light microscopy) and that other macromolecules such as albumin and PVP enhanced IgG transport by nonspecifically stimulating micropinocytosis.