A genetic locus from Campylobacter jejuni 81-176 (O:23, 36) has been characterized that appears to be involved in glycosylation of multiple proteins, including flagellin. The lipopolysaccharide (LPS) core of Escherichia coli DH5α containing some of these genes is modified such that it becomes immunoreactive with O:23 and O:36 antisera and loses reactivity with the lectin wheat germ agglutinin (WGA). Site-specific mutation of one of these genes in the E. coli host causes loss of O:23 and O:36 antibody reactivity and restores reactivity with WGA. However, site-specific mutation of each of the seven genes in 81-176 failed to show any detectable changes in LPS. Multiple proteins from various cellular fractions of each mutant showed altered reactivity by Western blot analyses using O:23 and O:36 antisera. The changes in protein antigenicity could be restored in one of the mutants by the presence of the corresponding wild-type allele in trans on a shuttle vector. Flagellin, which is known to be a glycoprotein, was one of the proteins that showed altered reactivity with O:23 and O:36 antiserum in the mutants. Chemical deglycosylation of protein fractions from the 81-176 wild type suggests that the other proteins with altered antigenicity in the mutants are also glycosylated.