We have partially characterized surface glycoproteins of the canine heartworm, Dirofilaria immitis. Histochemical studies indicated the presence of neutral and acidic mucopolysaccharides at the blood-cuticle interface. Fluorescein isothiocyanate-conjugated lectin binding patterns suggested the presence of alpha-D-glucosyl and/or alpha-D-mannosyl, beta-galactosyl, N-acetylneuraminyl and N-acetylated-D-hexosaminyl (sialic and glucuronic acids, respectively) terminal residues among the constituent sugars of the glycocalyx. An additional goal of this study was to assess the significance of each carbohydrate in parasite hemocompatibility by using scanning electron microscopy, internal reflection infrared spectroscopy, and comprehensive contact angle measurements. Each carbohydrate identified in the glycocalyx was selectively cleaved with the appropriate exoglycosidase. Heart-worms bearing native and enzyme-altered surfaces were exposed to platelet-rich canine plasma. Activation and aggregation of platelets were significantly increased on enzyme-treated surfaces as compared with native surfaces. Enzyme-induced cleavage of carbohydrate residues was associated with an increase in critical surface tension or a loss in cuticular structural integrity or both. Hemocompatibility of the heartworm cuticle depends on the retention of a stable saccharide-rich layer that minimizes interaction with plasma proteins and platelets; thus, carbohydrate residues on the glycocalyx may contribute to parasite hemocompatibility. The presence of similar low-critical-surface-tension coatings with high mechanical integrity may impart thromboresistance to other polyphenolic or chitinous substances.