The receptor which recognizes glycoproteins that have had their terminal sialic acids removed, thus exposing penultimate galactose residues (asialoglycoproteins), was examined for expression in rat liver during development. The level of asialoglycoprotein receptor binding activity in fetal rat livers was present in very low amounts but rose dramatically at the time of birth and reached adult levels by the second day after birth. Using immunoquantitation methods, it was found that the increased binding capacity of rat liver for asialoglycoproteins during development reflected accumulation of receptor molecules rather than activation of previously existing ones. The relative rates of synthesis of the predominant polypeptide of Mr 42,000 and the lesser abundant polypeptides of Mr 50,000 and 58,000 which comprise asialoglycoprotein receptor were found to increase in livers of fetuses near term and attain adult synthesis rates around birth. Thus, the accumulation of receptor protein molecules during development reflected increased synthesis of receptor polypeptides. These results suggest that the different gene products which code for the three forms of the receptor are coordinately expressed during development. Copurifying with asialoglycoprotein receptor during ligand affinity chromatography were polypeptides of Mr 25,000 and 27,000. These polypeptides display several characteristics similar to hepatic mannose binding lectin described by others. Onset of synthesis of the mannose binding lectin during development was analogous to asialoglycoprotein receptor but, in contrast, did not reach adult synthesis rates immediately after birth.