Monoclonal antibodies have been produced with both type-specific and broadly cross-reacting properties against the variant pili of Neisseria gonorrhoeae P9. Competitive radioimmunoassays demonstrated that at least three separate epitopes contribute to type-specificity within the strain. ELISA using purified pili from clinical isolates showed that some of the type-specific epitopes are randomly distributed among variants of single strains isolated from different sites in sexual partners. Cross-reacting antibody SM1 recognized an epitope present on all gonococcal isolates. This epitope was also present on a large proportion of meningococci and could be directly demonstrated in CSF from a patient with meningococcal meningitis. Commensal Neisseria and other Gram-negative bacteria tested failed to react. The meningococci which lack the epitope recognized by the antibody SM1 were nevertheless shown by electron microscopy to be pilated, suggesting that pili from meningococci may be antigenically more diverse than those from gonococci.