Summary The techniques of capsular serotype, biotype determination and sodium dodecylsulphate polyacrylamide gel electrophoresis of sarcosinate-insoluble outer membrane protein (OMP) and of proteinase K lipopolysaccharide (LPS) preparations were applied to 41 genital and neonatal Haemophilus influenzae isolates. Twelve percent were capsulated (4b, la). Distribution of strains between biotypes was similar to that of isolates of other non-systemic pathogenic origin; only one isolate was biotype IV. The OMP profiles showed great variability with 4 group of proteins: a 16-Kd major peptide which was observed in all strains; 27-30-Kd major OMP including one constant 30-Kd peptide present in all strains except one; 32− to 50-Kd major OMP; and 49− to 54-Kd minor OMP. The rough LPS profiles also revealed heterogeneity. In view of the variability observed among H. influenzae strains, it is not possible to establish a relationship between pathogenicity and a macromolecular marker.