To monitor temporal trends and regional differences in antibiotic susceptibility, we measured the minimum inhibitory concentrations for penicillin G, ampicillin, tetracycline, and spectinomycin of 4405 pre-treatment gonococcal isolates from patients with uncomplicated gonorrhea. As compared to isolates studied in 1970-1971, recent United States isolates appeared equally sensitive to penicillin G and more sensitive to tetracycline. Relatively resistant strains were found throughout the country. We also studied 1974 patients, treated for uncomplicated gonorrhea according to the 1972 regimens recommended by the United States Public Health Service, to determine the relation between pretreatment minimum inhibitory concentrations and treatment results. For patients receiving the procaine penicillin-probenecid and ampicillin-probenecid regimens, minimum inhibitory concentrations to the treatment drugs were significantly higher in patients not cured than in those cured (P less than 0.01 fr penicillin and P less than 0.05 for ampicillin). In contrast, spectinomycin-treatment results appeared to be independent of the isolate's susceptibility to spectinomycin and other antibiotics.