Ninety-five percent of systemic Haemophilus influenzae infections in childhood are caused by serotype b organisms. The high risk for mortality and serious sequelae and the increase in antibiotic resistance of H influenzae type b are strong arguments for vaccination. Incidences and age distribution of disease caused by H influenzae type b vary considerably among countries. Compiled data for meningitis show the highest incidences to be among Alaskan Eskimos, Navajo and White Mountain Indians, and aboriginals in Australia. High incidences coincide with a peak incidence in a younger age group. This differs from Finland, where 95% of the cases of disease caused by H influenzae type b occur in children older than 7 months of age. In general, incidences are low in industrial and high in nonindustrial areas. For example, in Gambia the highest age-specific incidence is at 4 to 5 months after birth (H.A.B., unpublished data, 1988). This implies that a vaccine and regimen similar to that used in Finland would not be as efficacious if used in Gambia because of differences in demographics and incidence. Vaccines that confer protection at 3 to 4 months of age are, therefore, strongly desired. Outer membrane proteins and lipopolysaccharides of H influenzae type b have been suggested as alternative vaccine candidates in conjugation with the capsular polysaccharide because they apparently can contribute to the virulence of H influenzae type b. The occurrence and immunogenicity of various outer membrane proteins and lipopolysaccharides among H influenzae type b in industrial and nonindustrial countries and their significance as epidemiologic markers for the spread of disease, the type of disease, the age of acquisition, and their association with antibiotic resistance will be reviewed in this article.(ABSTRACT TRUNCATED AT 250 WORDS)