Summary. Rabbit antiserum and murine monoclonal antibodies were raised against a strain of Haemophilus ducreyi. The antiserum gave high immunofluorescence titres and strong dot blot reactions with all H. ducreyi strains tested and the only cross reaction was with Bordetella pertussis. Three monoclonal antibodies, all of isotype IgG2a, also gave high immunofluorescence titres with H . ducreyi but did not cross react with any other species tested. Immunoblotting showed the monoclonal antibodies to react with a single polypeptide band of mol. wt 29000 in the outermembrane fraction of H . ducreyi. These antibodies have potential for use as diagnostic reagents and for investigating the pathogenicity of H. ducreyi.
Introdnc tionHaemphilus ducreyi is the causative organism of the sexually transmitted disease chancroid, the commonest cause of genital ulcers in the tropics. However, in recent years there have been increasing numbers of outbreaks in the more temperate regions of Europe and North America. 92 H . ducreyi is a fastidious organism and, despite improved culture methods, isolation rates from genital ulcers and buboes can still be lower than 60%. In many countries where facilities for culture are often not available, diagnosis of chancroid continues to be based on clinical features and the exclusion of other causes of genital ulcers. This form of diagnosis is prone to e r r~r .~ Direct microscopy of gram-stained smears is also insensitive; the typical "shoal of fish" morphology of H . ducreyi is often not seen, and other contaminating gram-negative bacilli may look ~i m i l a r .~?~To overcome these problems, several groups of workers have attempted to develop immunological methods of diagnosis. Denys et al. ' raised an antiserum for use in an immunofluorescence assay for H. ducreyi. This antiserum, however, had to be absorbed extensively with other Haemophilus spp. to eliminate cross-reactivity. Even after absorption it gave a moderate fluorescence with several other Haemophilus spp., in particular H . parainfluenzae. In 1984, Hansen and Loftus6 raised monoclonal