The effect of molecular charge on the permeability to macromolecules of the fetal capillary endothelium of the guinea-pig placenta has been investigated. In a series of experiments for horseradish peroxidase (HRP) isoenzymes with different isoelectric points were perfused through the fetal circulations of guinea-pig placentae, which were then prepared for ultracytochemical localization of peroxidase activity. It was found that the two anionic HRP tracers did not penetrate the endothelial cell layer of the fetal capillaries. By contrast, both cationic HRP tracers did penetrate; reaction product was found in the subendothelial space underlying the endothelial cell layer. The route of transendothelial penetration was via the lateral intercellular spaces (LIS) between endothelial cells. In further experiments cationic and anionic ferritins were perfused. Cationic ferritins were found apparently bound in clusters to the luminal surface of the endothelial cell layer in the luminal ends of the LIS whereas very few anionic ferritin molecules were present. We conclude that molecular charge is an important determinant of macromolecule permeability through the fetal capillary endothelium of the guinea-pig placenta and may be related to the presence of anionic sites on the luminal plasma membrane of the endothelial cells.