Monoclonal antibody AM-3 (MAb AM-3) raised against a sialomucin from human colorectal carcinoma has previously been shown to define a carbohydrate epitope, which is detectable by immunocytochemistry on all investigated colonic carcinomas and is expressed in correlation with the grade of dysplasia in colonic adenomas (Hanski et al., J. Clin. Pathol., 43: 379-385, 1990). Epitope analyses in solid-phase enzyme immunoassays revealed that AM-3 antibody recognizes the sialylated Lewis(x) sequence on a branched O-linked glycan and its reductively cleaved alditol from human amniotic mucins. In comparative binding and binding inhibition studies MAbs AM-3 and CSLEX1 displayed reciprocal affinities to mucins versus gangliosides. Correspondingly, the weaker binding activities of AM-3 versus CSLEX to III3-alpha Fuc-IV3-alpha NeuAc-nLcOse4-Cer or to monogangliosides from human granulocytes were measured. Gangliosides from a human colon carcinoma were recognized by MAb CSLEX1 exhibiting a broader specificity to various sialyl-Lewis(x) antigens and by MAb FH6 reactive to sialyl-dimeric Lewis(x) antigen, but not by MAb AM-3. In conclusion, MAb AM-3 is distinguished from other sialyl Lewis(x)-specific MAbs by its selective reactivity to mucin-carried epitopes on the monomeric antigen.