The major objective of this research proposal is to continue to investigate immunity to infection with "Hemophilus influenzae," type b. Among the areas being studied are the patterns of colonization with "Hemophilus influenzae," type b, in the families of individuals who had infection with this organism, the immunologic responses induced by this organism in the families and contacts of patients infected with "Hemophilus influenzae," type b, investigation of the effects of immunization against "Hemophilus influenzae," type b, in older children who have previously had meningitis with this organism, and investigation of the immunology of experimental "Hemophilus influenzae," type b, meningitis in rats. Because of the widespread interest in the possibility of immunization of young children with a polysaccharide vaccine protective against "Hemophilus influenzae," it seems important to understand the immune defense mechanisms, both natural and induced by immunization, which protect against infection with "Hemophilus influenzae, type b. One of the major goals of this research is to better clarify the relationship between the immune response and the age of the affected individual. It is hoped that the use of an animal model will further elucidate this problem. BIBLIOGRAPHIC REFERENCES: Norden, C, Michaels, RH, Melish, M: Effect of previous infection on antibody response of children to vaccination with capsular polysaccharide of "Haemophilus influenzae," type b. J Infect Dis 132:69, 1975. Norden, C and Feldman, H: "Hemophilus influenzae," type b, antibody frequencies determined with bactericidal and radioimmunosssay tests. J Clin Microbiology 2:136, 1975.