1-Ethyl-1,4-dihydro-4-oxo-7-(4-pyridyl)-3-quinoline carbonic acid (rosoxacin, Win 35 213) is a new broad-spectrum pyridyl quinoline derivative. Its activity in vitro against 247 beta-lactamase-negative and 9 beta-lactamase-positive strains of Neisseria gonorrhoeae was compared with those of penicillin, ampicillin, cefaclor, spectinomycin and tetracycline, using the microtiter-dilution technique. Rosoxacin proved to be approximately equal in activity to penicillin against penicillin-sensitive isolates of N. gonorrhoeae. Against beta-lactamase-producing isolates rosoxacin was the most active compound tested: all strains were inhibited by 0.06 mg/l.