The susceptibility of 19 isolates of Haemophilus ducreyi from a recent chancroid outbreak and four reference strains was determined in vitro to 13 antimicrobial agents. The rabbit intradermal test for virulence was positive for all of the local isolates, but not for the reference strains. The "nonvirulent" reference strains were inhibited by lower minimum inhibitory concentrations (MICs) of most agents tested. For the virulent isolates, the range of MICs (in micrograms per milliliter) of the following were: of vancomycin, 8 to 128; of polymyxin, 32 to 128; of cloxacillin, 32 to 64; of tetracycline, 0.5 to 32; of cephalothin, 4 to 8; of doxycycline, 0.25 to 8; and of kanamycin, 1 to 8. Three strains were resistant to penicillin and ampicillin (MIC >/= 128 mug/ml), and these three strains produced beta-lactamase. The remainder were susceptible to 4 mug/ml. All strains were susceptible to rifampin (MIC </= 1 mug/ml), chloramphenicol (MIC </= 4 mug/ml), sulfisoxazole (MIC </= 8 mug/ml), and nalidixic acid (MIC </= 8 mug/ml). These susceptibilities of H. ducreyi indicate several antimicrobial agents that may be effective for chancroid treatment and support the use of vancomycin in a selective medium for the culture of chancroid genital ulcers.