The mouse and rabbit intradermal injection models have been used to define factors that may be important in Haemophilus ducreyi pathogenesis. We used H. ducreyi strains with diverse geographic origins and phenotypic characteristics to evaluate the experimental models. Injection of live and heat-killed bacteria caused skin abscesses in both models. Semiquantitative cultures of skin injected with live bacteria showed that H. ducreyi failed to replicate in animal tissue. These data suggested that the experimental lesions were caused by a heat-stable substance such as lipooligosaccharide (LOS). In mice, injection of H. ducreyi and Haemophilus influenzae LOS and Escherichia coli lipopolysaccharide caused mild to moderate inflammation. In rabbits, injection of H. ducreyi LOS caused intradermal abscesses that were histologically similar to those caused by live and heat-killed bacteria. H. ducreyi and Neisseria gonorrhoeae LOS caused significantly larger lesions than equivalent amounts of H. influenzae LOS and E. coli lipopolysaccharide in the rabbit model. We conclude that the intradermal injection models are not valid models to study the growth of H. ducreyi in vivo. However, these data indicate that H. ducreyi LOS may play an important role in the pathogenesis of chancroid and that the rabbit model should be useful in studying H. ducreyi LOS toxicity at the cellular level.