Nontypable Haemophilus influenzae has now become well established as an important pathogen in both adults and children. Recent work has identified clear distinctions between nontypable and type b strains of H. influenzae. These organisms affect different patient populations, cause different infections, present different surface antigens to the host, and are genetically different. The commonest clinical manifestation of infection due to nontypable H. influenzae in adults is lower respiratory tract infection, particularly in the elderly and in those with chronic bronchitis. The bacterium is a frequent cause of acute otitis media in children. The surface of nontypable H. influenzae is composed of outer-membrane proteins and lipooligosaccharide, and both of these demonstrate substantial antigenic heterogeneity, which can be used to serotype isolates. Some respiratory tract isolates are fimbriated, but the role of fimbriae in pathogenesis is unclear. Antibodies to outer-membrane proteins and lipooligosaccharide are present in human serum. Investigation of human immunity to infection is focusing on identification of those antigens to which protective antibody is directed.