S tructural relationships between colonic mucin species were assessed using a library of monoclonal antibodies (MAbs) directed against purified human colonic mucin (HCM). After immunization of mice with purified mucin from normal human colonic mucosa, 14% of 1,920 fusion products screened were positive for anti-HCM activity in a solid-phase assay. Patterns of selective binding by hybridomas to six discrete HCM species (I-VI) separated by DEAE-cellulose chromatography suggested the presence of both shared and species-specific antigenic determinants among HCM species I-VI. 23 anti-HCMs MAbs (7 IgM, 7 IgG1, and 9 IgG2) demonstrating a range of anti-HCM species specificities, were produced and used to study structural relationships between mucin species. Binding of various mucin species by individual anti-HCM MAbs was shown by competitive solid-phase radioimmunoassay to reflect the presence of identical epitopes on the different species. Adsorption of HCM species on a variety of affinity resins prepared with anti-HCM MAbs demonstrated that binding to multiple mucin species by a single MAb was related to intrinsic structural determinants. Four anti-HCM MAbs recognized protease-sensitive antigenic structures, which suggests that they may be directed to core HCM proteins. 12 of the anti-HCM MAbs were shown by solid-phase assay to recognize either complete (n = 5) or partial (n = 7) isolated colonic mucin oligosaccharide side chains of defined structure. Collectively, these data show the presence of both shared and unique antigenic structural determinants among colonic mucin species. Chromatographic heterogeneity of mucin glycoproteins seems to be related to the existence of biologically significant subclasses in the normal human colon.