Protegrins, potent antimicrobial peptides found in porcine leukocytes, have activity against the sexually transmitted pathogens Neisseria gonorrhoeae, Chlamydia trachomatis, and human immunodeficiency virus type 1. We tested synthetic protegrin 1 (PG-1) for activity against nine isolates of Haemophilus ducreyi, the etiologic agent of chancroid. The test organisms included CIP 542 (the type strain), 35000HP (a human-passaged variant of 35000), 35000HP-RSM2 (an isogenic D-glycero-D-manno-heptosyltransferase mutant of 35000HP), and six clinical isolates. The isolates were epidemiologically unrelated, represented three HindIII ribotypes, and had varying antimicrobial resistance patterns. In bactericidal assays, five isolates were rapidly killed by synthetic PG-1. In radial diffusion assays, all nine isolates were exquisitely sensitive to PG-1. These data highlight the potential of protegrins for development as topical agents to prevent many sexually transmitted diseases, including chancroid.