Human antithrombin III was found to contain covalently linked N-acetylglucosamine, mannose, galactose, and sialic acid in a molar ratio of approximately 1:1:0.6:1. Sialic acid was released upon treatment with neuraminidase. The modified glycoprotein retained the capability to inhibit thrombin and to bind with heparin. Antithrombin III isolated by different procedures was also found to contain glucose in an approximately equimolar ratio with N-acetylglucosamine. Th" glucose-containing component was extractable with lipid solvents and shown to be beta-glucosylceramide. This glycolipid is tightly complexed with antithrombin III and could not be separated by fractional precipitations or ion exchange gels. Although it remains to be established whether the inhibitory actions of antithrombin III are affected by glucosylceramide, the relative amounts which are bound suggest that antithrombin III may be a significant carrier of the glycolipid.