Strains of Haemophilus influenzae type b were collected as part of an epidemiological study of bacterial meningitis in the North East Thames Region (NETR) of England. Subclones of H. influenzae were identified by outer-membrane protein typing and multilocus enzyme electrophoresis. These were compared with subclones of H. influenzae type b isolated from elsewhere in the UK. The subclone ET 12.5/OMP 3L was identified in 68% of isolates from cases of meningitis from the NETR. In the isolates from elsewhere in the UK, this subclone was identified in 79% of sterile-site isolates and 91% of isolates from non-sterile sites. This subclone predominated in patients of different ages and from different ethnic groups, which suggests that non-Caucasians are infected with the subclone predominating locally. Since ET 12.5/OMP 3L subclone is the most abundant strain isolated in virtually all other Western European countries, these results provide additional evidence that the genetic diversity in the H. influenzae type b population in Europe is relatively restricted.