Oligonucleotides that correspond to regions of the penicillin-binding protein 2 gene (penA) that differ between penicillin-sensitive and penicillin-resistant strains have been used as probes to classify the penA genes in a collection of penicillin-resistant gonococci isolated in Britain. 44/47 of those gonococcal strains that had minimal inhibitory concentrations of greater than or equal to 0.25 microgram benzylpenicillin per ml contained extensively altered penA genes which appeared to be very similar (or identical) to one or other of the two classes of altered penA genes that have been described previously. Since these two classes of altered penA genes are related, it appears that the great majority of the altered penA genes on non-beta-lactamase-producing penicillin-resistant gonococci have a clonal origin. The other three penicillin-resistant strains had altered penA genes that were different to those described previously. A crucial step in the development of the altered forms of PBP2 with decreased affinity for penicillin appears to have been the insertion of an extra codon within the transpeptidase domain of the penA gene. This insertion was found in the penA gene of all gonococci with minimal inhibitory concentrations of greater than 0.016 microgram benzylpenicillin per ml but was not found in any strains with minimal inhibitory concentrations of less than or equal to 0.016 microgram per ml.