The aim of this study was both to investigate the in-vitro susceptibility to thiamphenicol of Neisseria gonorrhoeae strains isolated at different times in different geographic areas and to compare results of susceptibility testing by the agar-dilution method with those of an agar disk-diffusion test. For 209 strains studied, a strong correlation between MIC values and zone diameters could be demonstrated (Spearman Rank correlation coefficient, -0.77; P less than 0.0005). The total material consisted of 727 selected gonococcal strains: 184 from Denmark (1928-1982), 55 from Africa (1960-1966), 47 from France (1981-1982), 236 from Greenland (1982), and 205 from Indonesia (1982). Danish strains from the preantibiotic era (1928-1940) were all susceptible to less than or equal to 1.0 microgram of thiamphenicol/ml. In 1982, the less susceptible strains (MIC, greater than or equal to 2 micrograms/ml; or zone diameter, less than or equal to 45 mm) accounted for 5% (Denmark), 28% (Greenland), 28% (France), and 82% (Indonesia), respectively. All non-penicillinase-producing strains of N. gonorrhoeae that were less susceptible to thiamphenicol were also less susceptible to penicillin (MIC, greater than or equal to 0.6 microgram/ml).