A lectin that can specifically bind to O-acetylsialic acids, found in glycoproteins and gangliosides, was purified to homogeneity from a crab Cancer antennarius (crab lectin) (Ravindranath, M. H., Higa, H. H., Cooper, E. L., and Paulson, J. C. (1985) J. Biol. Chem. 260, 8850-8856; Correction (1986) J. Biol. Chem. 261, 1983; Ravindranath, M. H., and Paulson, J. C. (1987) Methods Enzymol. 138, 520-527). We tested lectin binding to human melanoma cell lines to identify O-acetylsialylated gangliosides on the melanoma cell surface. The highest degree of binding of the crab lectin was demonstrated on a melanoma cell line, UCLASO-M25. To confirm that the binding was due to O-acetylsialic acid in the alkali-labile gangliosides, the gangliosides were isolated and purified from M25 cells and individually coated onto sheep asialoerythrocytes, which served as targets in an agglutination assay using the lectin. The crab lectin agglutinated the asialo-sheep erythrocytes coated with alkali-labile gangliosides, but the lectin failed to agglutinate the asialoerythrocytes coated with GM3, GD3, and base-treated gangliosides. Subsequently, the purified alkali-labile M25 ganglioside was base-treated and applied to TLC, and we found that it was converted to a slower migrating species identical to the disialolactosylceramide (GD3). These results indicate that O-acetyl GD3 expressed on the melanoma cell surface is recognized by the lectin. Because O-acetyl GD3 is not expressed on human normal tissues, we examined the capability of O-acetyl GD3 to induce immune responses in man. Sera from patients with melanoma were tested against M25 cells in an immuneadherence assay, and those positive to the M25 cell line were further tested for specificity to O-acetyl gangliosides. The presence of autoantibodies to O-acetyl-GD3 in melanoma sera was confirmed by blocking of the antigen sites on M25 cells by the lectin or preabsorption of the sera with erythrocytes bearing O-acetyl gangliosides. The data provide evidence that O-acetyl-GD3 may represent an important tumor marker for detection and treatment of human melanoma.