Eighty strains of Haemophilus influenzae type b were randomly selected from 531 strains collected between 1975 and 1982 from patients with meningitis in The Netherlands. Subtyping by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that 67 of 80 isolates had identical major outer membrane protein patterns (subtype 1). Among the 13 other isolates four different polyacrylamide gel electrophoresis patterns were observed, two of which closely resembled subtype 1. Lipopolysaccharides were characterized immunologically by immunoprecipitation (Ouchterlony technique) and the gel-immune-radio-assay. Four serotypes were found among the 80 selected strains, leaving one strain not typable. Seventy-four strains (93%) belonged to the same lipopolysaccharide serotype; 77 (97%) of 80 of the strains belonged to biotype I. Sixty strains (75%) had identical major outer membrane protein patterns (subtype 1), lipopolysaccharide serotypes (serotype 1), and biotypes (I).