Penicillin-resistant (penr) clinical isolates of Neisseria meningitidis, which do not produce beta-lactamase, were first identified in Spain in 1985; the frequency of their recovery, which has been increasing in the past few years, reached 20% in 1989. Serogrouping, determination of serotypes and subtypes, and multilocus enzyme electrophoresis of the penr strains showed an extensive diversity. Resistance is due, at least in part, to a decreased affinity of penicillin-binding protein (PBP) 2 for penicillin. Similar low-affinity forms of PBP 2 are also found in penr isolates of Neisseria lactamica, Neisseria polysaccharea, and Neisseria gonorrhoeae. Genetic transformation of an N. meningitidis type strain to low-level penicillin resistance with DNA from resistant meningococci and other Neisseria species resulted in transformants that possessed low-affinity forms of PBP 2. These altered forms of PBP 2 have been shown to arise from recombinational events that replace parts of the PBP 2 gene with the corresponding regions from the PBP 2 genes of commensal Neisseria species.