Haemophilus ducreyi35000HP contains a cluster of homologues of genes required for the synthesis of enterobacterial common antigen (ECA), suggesting thatH. ducreyimay express a putative ECA-like glycoconjugate. WecA initiates the synthesis of ECA by transferring N-acetylglucosamine to undecaprenyl-P, to form lipid I. A wecA mutant (35000HPwecA) was constructed, and 5 volunteers were inoculated at 3 sites with fixed doses of 35000HP on one arm and at 3 sites with varying doses of 35000HPwecAon the other arm. 35000HPwecAcaused pustules to form at 3 sites inoculated with a dose 2.5-fold higher than that of 35000HP. However, at sites inoculated with similar doses of 35000HP and 35000HPwecA, pustules developed at 46.7% (95% confidence interval [CI], 23.3%- 70.0%) of 15 parent-strain sites and at 8.3% (95% CI, 0.01%-23.6%) of 12 mutant-strain sites (P= .013). Thus, the expression ofwecAcontributes to the ability ofH. ducreyito cause pustules in humans.