The saccharidal constituents of the surfaces of hemocytes of the American oyster, Crassostrea virginica, from Apalachicola Bay, Florida, and Galveston Bay, Texas, were determined by employing eight lectins with known sugar specificities (Con A, Tetragonolobus purpureas, Limulus polyphemus, Dilichos biflorus, Sambucas nigra, Glycine max, Triticum vulgaris, and Lathyrus odoratus). Known inhibiting sugar residues were used to inhibit clumping of lectin-treated oyster hemocytes. As a result, the following saccharides were demonstrated to occur on oyster hemocytes: N-acetyl-D-glucosamine, N-acetyl-D-galactosamine, methyl-alpha-D-mannopyranoside, D(+)-glucose, sucrose, D(+)-mannose, alpha-methyl-D-galactoside, beta-D(-)-fructose, L(-)-fucose, N-acetylneuraminic acid, and D(+)-galactose. In addition, D-glucuronic acid occurred only on oyster cells from Apalachicola Bay and an unidentified sugar that binds to the L. odoratus lectin that is neither D(+)-glucose nor D(+)-mannose, the usual inhibiting sugars, occurs on hemocytes from both Apalachicola and Galveston Bays. Quantitative and qualitative differences in the saccharidal constituents on the surfaces of hemocytes from oysters from the two collecting sites are attributed to strain differences