Twenty-four Haemophilus influenzae type b strains from 836 children and young adults in an open population were subtyped by outer-membrane-protein (OMP) analysis on sodium dodecyl sulphate-polyacrylamide gels, lipopolysaccharide serotyping and biotyping. The results were compared with those obtained with H. influenzae type b strains from 97 patients with meningitis in the same city (Amsterdam). OMP subtype 1 was significantly more common among the CSF isolates than in carrier strains (82% vs 50%; p less than 0.002). The other OMP subtypes found among carriers were rarely isolated from patients. The lipopolysaccharide serotype and biotype distribution did not differ between the two groups. The combination of OMP subtype 1, lipopolysaccharide 1, biotype I was much more common in isolates from patients than in those from carriers (71% vs 42%; p less than 0.01). The data suggest that various H. influenzae type b subtypes are less virulent than those commonly isolated from invasive infections.