We isolated two independent mutations in Dictyostelium discoideum that result in the absence of the antigenic determinant recognized by monoclonal antibody E28D8. This antibody reacts with a post-translational modification on the surface glycoprotein gp80 and several other proteins. Both of the mutations occur in the same locus, modB, which was mapped to linkage group VI. The modB mutations result in sufficient alteration of gp80 that it is absent or unrecognizable by two-dimensional gel electrophoresis. Strains carrying modB mutations exhibit "contact sites A"-mediated cell-cell adhesion although more weakly than do wild-type strains and develop to fruiting bodies carrying viable spores. Although gp80 has been implicated in the mechanism of cell-cell adhesion in D. discoideum, it is clear from the behavior of these mutant strains that the determinant on gp80 recognized by E28D8 is not necessary for either morphogenesis or reduced EDTA-resistant adhesion.