OBJECTIVE--To study the changes in penicillinase-producing (PPNG) and high-level tetracycline resistant (TRNG) Neisseria gonorrhoeae isolated in Hong Kong associated with emerging quinolone resistance (QRNG) over a two year period from November 1992 to October 1994. MATERIALS AND METHODS--Four thousand and eighty-six strains of Neisseria gonorrhoeae isolated, of which 432 were PPNG, were examined for susceptibilities to penicillin and tetracycline by an agar dilution method using the breakpoint minimum inhibitory concentrations (MICs) of 1 and 10 mg/1 respectively. Ofloxacin susceptibility was done using 0.1 and 1 mg/l. Penicillinase production was detected by performing the chromogenic cephalosporin nitrocefin test on all penicillin resistant (MIC > 1 mg/l) strains. RESULTS--Three thousand and eighty (75.4%) and 79 (1.9%) strains were found to be penicillin resistant and TRNG (MIC > 10 mg/l) respectively. Sixty-nine strains (1.7%) were resistant to both, of which 54 (1.3%) were PPNG. Three strains were multiply-resistant to penicillin, tetracycline and ofloxacin; however, none was PPNG. While the percentage of penicillin resistant strains remained stable (mean 75.5%, SD 7.0), TRNG decreased from 4.5% to 2.1%. The most dramatic change was the sharp decline of PPNG from 25.5% in January 1993 to 4.3% in October 1994, concurrent with a linear increase in strains with ofloxacin MIC > 0.1 mg/l. Significant clinical failure was seen in strains having ofloxacin MIC > 1 mg/l (QRNG), which increased drastically from 0.5% to 10.4% during the study period. Selection against PPNG and TRNG strains appeared to occur only when fully quinolone-susceptible strains first become less susceptible (MIC > 0.1 mg/l), but not when these less susceptible strains become fully resistant (MIC > 1 mg/l). CONCLUSION--PPNG is now no longer hyperendemic in Hong Kong. Emergence of QRNG is associated with rapid decline of both PPNG and TRNG. This is the first report of plasmid-curing effect of the 4-fluoroquinolones occurring on an ecological scale.