Phosphorylcholine (ChoP) is a component of the teichoic acids ofStreptococcus pneumoniae and has been recently identified on the lipopolysaccharide of Haemophilus influenzae, also a major pathogen of the human respiratory tract. Other gram-negative pathogens that frequently infect the human respiratory tract were surveyed for the presence of the ChoP epitope as indicated by binding to monoclonal antibodies (MAbs) recognizing this structure. The ChoP epitope was found on a 43-kDa protein on all clinical isolates ofPseudomonas aeruginosa examined and on several class I and II pili of Neisseria meningitidis. The specificity of the anti-ChoP MAb was demonstrated by the inhibition of binding in the presence of ChoP but not structural analogs. As in the case of H. influenzae, the expression of this epitope was phase variable on these species. In P. aeruginosa, this epitope was expressed at detectable levels only at lower growth temperatures. Expression of the ChoP epitope on piliated neisseriae displayed phase variation, both linked to pilus expression and independently of fully piliated bacteria.