A previous study in our laboratory identified a surface-exposed peptidoglycan-associated protein of Neisseria gonorrhoeae which had an apparent molecular mass of 44000 daltons (44kDa) (Hill and Judd, 1989). This paper reports results which confirm that the 44 kDa protein is surface-exposed, and that the protein is expressed in, and is structurally invariant among, 14 strains of N. gonorrhoeae. The fact that the 44kDa outer-membrane protein is found in a conserved form in all gonococci examined strongly suggests that ft is crucial to the bacterium's survival. Moreover, it appears that this protein is a penicillin-binding protein (PBP3) (Shafer and Judd, 1991). This invariant, surface-exposed, peptidoglycan-associated outer-membrane protein deserves further investigation to elucidate its role in the immunobiology of N. gonorrhoeae, and its possible use as an immunoprophylactic reagent.