We monitored the development of serum bactericidal antibody in eight children with acute nontypable Haemophilus influenzae otitis media and correlated its development with the appearance of antibody against lipooligosaccharide and surface-exposed outer membrane proteins of the infecting strains. Complement-dependent bactericidal activity was absent in acute sera but increased to titers of 1:4 to 1:32 in sera obtained 4 to 6 weeks later. Absorption of anti-lipooligosaccharide antibodies from convalescent sera had no effect on bactericidal titers of five patients and resulted in small decreases in titer in three patients. Lipooligosaccharide-absorbed samples had persisting bactericidal titers of 1:4 to 1:16. Four of eight acute samples lacked antibodies to surface-exposed outer membrane proteins whereas four had low concentrations of antibody directed against one or more M*** 100 000 to 250 000 outer membrane proteins. Convalescent samples from all eight children showed substantial increases in antibodies directed primarily against M*** 100 000 to 250 000 proteins. Thus, both surface-exposed M*** 100 000 to 250 000 outer membrane proteins and lipooligosaccharide are immunogenic during Haemophilus otitis media and are potential targets of bactericidal antibody.