Increasing resistance of Haemophilus ducreyi to antimicrobials necessitates further trials of new antimicrobial agents for treating chancroid. Enoxacin has excellent in vitro activity against H ducreyi, and a randomised clinical trial of three doses of enoxacin 400 mg at intervals of 12 hours compared with a single dose of trimethoprim/sulphametrole (TMP/SMT) 640/3200 mg was therefore conducted. Of 169 men enrolled in the study, 86 received enoxacin and 83 received TMP/SMT. Ulcers were improved or cured in 65/73 men treated with enoxacin and 57/70 men treated with TMP/SMT. This difference was not significant. At 72 hours after treatment, H ducreyi was eradicated from ulcers of 72/77 men treated with enoxacin and of 67/74 of those treated with TMP/SMT. Patients with buboes responded equally well to both treatments. Of 100 H ducreyi strains tested, all were susceptible to both 0.25 mg/l enoxacin and the combination of 0.25 mg/l TMP and 5 mg/l SMT. Although most men treated with either regimen were cured, neither regimen appeared to be the optimum treatment for chancroid. This study shows the efficacy of enoxacin for a soft tissue infection caused by Gram negative organisms.