During the first 4 months of 1977, 95 different strains of gonococcus were isolated in the out-patient department of the Dermatology Clinic of the University of Basel. These were systemically tested for sensitivity to penicillin G, spectinomycin, thiamphenicol, doxycycline and rifampicin. The MIC values and the diameter of the zones of inhibition were determined. Tests for beta-lactamase production were made. A correlation in the degree of sensitivity was observed between the following antibiotics: penicillin G/rifampicin; penicillin G/thiamphenicol; penicillin G/doxycycline; thiamphenicol/doxycycline; thiamphenicol/rifampicin; doxycycline/rifampicin. The size of the inoculum had little influence on the activity of the substances, except for that of penicillin G against the beta-lactamase-positive reference strains. No beta-lactamase-producing strains were detected amongst the clinical isolates. In the case of penicillin G, there was a correlation between the minimum inhibitory concentration and the inhibition-zone diameter.