Three oligonucleotide probes, complementary to tetM sequences, were labelled non-radiometrically using the DIG-oligonucleotide tailing kit and evaluated for their specificity for the detection of plasmid mediated tetracycline resistance in Neisseria gonorrhoeae. Only Probe 3, 5'-GCT CAA CAA TTC TGT TCC AGC-3', was specific for tetM. It hybridized with the tetM-containing 25.2-MDa plasmids from all of the 232 TRNG and the 130 PP/TRNG isolates used in the study. Its sensitivity, determined by dot-blot hybridization, was 0.1 pg of pJ13 plasmid DNA or 10(4) cells. It did not hybridize with the DNA from non-PPNG, CMRNG and tetracycline susceptible isolates from seven other Neisseria species (N. meningitidis, N. subflava, N. cinerea, N. lactamica, N. sicca, N. mucosa, and N. flavescens), Moraxella spp. and Haemophilus influenzae. Probe 3 also hybridized to DNA of three tetracycline resistant P. magnus (MIC = 16 micrograms ml-1) isolates which presumptively carried the tetM determinant. Therefore, probe 3 can be used by reference laboratories as a confirmatory test for TRNG, as well as isolates from other genera containing the tetM determinant.