Susceptibility to thiamphenicol was tested, and plasmids were characterized for 1925 strains of penicillinase-producing Neisseria gonorrhoeae (PPNG) isolated in The Netherlands in 1981 and 1982. Of the 1925 isolates, 771 harbored the so-called "Asia" (4.5-megadalton) resistance plasmid and 1154 had the so called "Africa" (3.2-megadalton) plasmid. Isolates in the first group were usually inhibited by 0.25 microgram of thiamphenicol ml, while the MIC for the latter isolates was 1-2 micrograms/ml. A similar bimodal distribution of MICs has been observed for non-PPNG isolates. In the period covering this study, a substantial shift from predominance of the "Africa" type to that of the "Asia" type of N. gonorrhoeae was observed.