The techniques of biotype determination and sodium dodecyl sulfate-polyacrylamide gel electrophoresis of outer membrane protein preparations were applied to 35 epidemiologically unrelated isolates of pathogenic nontypable Haemophilus influenzae. Three of five isolates obtained from the blood of unrelated newborns with sepsis had concordant major outer membrane from the blood of unrelated older children or adults with bacteremia had concordant major outer membrane protein profiles, distinct from the common profile of neonatal strains, and were biotype II. The outer membrane protein profiles of the remaining 5 isolates from blood, 2 isolated from cerebrospinal fluid, and 23 isolated from middle ear aspirates of children with otitis media were unique, although each isolate had peptides with apparent molecular weights of 16,000 and 31,500. These results suggest that a subset of nontypable isolates associated with bacteremia has distinctive strain markers. Their pathogenicity may relate to a prediction for colonizing the female genital tract in the case of the common neonatal strain or an increased ability to evade host defenses.