Adsorptive endocytosis of bovine beta-glucuronidase by human fibroblasts is mediated by two different membrane receptors: one recognizes phosphomannosyl residues on the enzyme, the other is yet a undefined recognition marker (A. González-Noriega, R. Coutiño, V. M. Saavedra, and R. Barrera (1989) Arch. Biochem. Biophys. 268, 649-658). We have purified a bovine liver inhibitor for the endocytosis of the bovine beta-glucuronidase mediated by the recently proposed recognition marker. The inhibitor is partially susceptible to periodate oxidation, can be released from a peptide backbone by mild alkali treatment, can be reduced by sodium borohydride, and can be adsorbed to anionic but not to cationic resins. Although the chemical structure of the isolated marker has not been determined, results indicate a 122-Da molecule which may contain amino alcohol groups and may be found in a 1800-Da glycosidic chain.