Bacteria of the family Pasteurellaceae are well known to exist in close relationship with mammalian hosts, mostly as constit- uents of commensal bacterial flora. However, at the same time, some species also have the ability to cause serious diseases. For example, Mannheimia haemolytica and Haemophilus somnus cause pneumonia in cattle (15); Pasteurella multocida is a com- mon animal pathogen but occasionally causes meningitis in humans (40, 84); and Actinobacillus actinomycetemcomitans, although a commensal inhabitant of the oral cavity in humans, is also the leading agent of juvenile periodontitis (60), and in progressive dental disease, it can cause endocarditis (63). Other Actinobacillus spp., such as A. pleuropneumoniae, A. lignieresii, A. suis, and A. hominis, cause disease in animals. In humans, Haemophilus parainfluenzae and Haemophilus influ- enzae account for about 10% of the constant flora of the healthy upper respiratory tract, with H. parainfluenzae being more frequent but less pathogenic than H. influenzae. In par- ticular, H. influenzae is capable of causing serious respiratory infections, and until the introduction of conjugative vaccines against the capsule, it was the leading cause of bacterial men- ingitis in young children (53). Finally, Haemophilus ducreyi can be isolated from the genital mucosa of patients suffering from chancroid disease, which is prevalent predominantly in devel- oping countries (7, 78, 81).