The major outer membrane protein of Haemophilus influenzae type b (Hib) with an apparent molecular weight of 39,000 (39K) was purified from three different Hib strains and was shown to be free from detectable contamination with other proteins. However, these purified 39K protein preparations were found to contain Hib lipopolysaccharide (LPS). Immunization of rats with these 39K protein preparations resulted in the production of antisera containing both 39K protein-directed and LPS-directed antibodies, as determined by Western blot analysis. The reactivity pattern of the LPS-directed serum antibodies with different Hib strains was identical to the reactivity of these Hib strains with a set of monoclonal antibodies (mabs) previously shown to immunoprecipitate the 39K protein in a radioimmunoprecipitation (RIP) system. Examination of the antigenic specificities of the 39K protein-immunoprecipitating mabs by using Western blot analysis showed that these mabs were actually directed against Hib LPS. RIP analysis of 125I-labeled Hib cells and 32P-labeled Hib cells revealed that the 39K protein and LPS existed as a complex in a RIP system, which resulted in the coprecipitation of both antigens by LPS-directed mabs. The interaction between LPS and the 39K protein was highly selective for this protein and did not involve other outer membrane proteins. The LPS/39K protein complex could be reconstituted by mixing purified LPS and purified 39K protein; it could also be reconstituted with 39K protein from one Hib strain and LPS from another Hib strain. These findings have necessitated the reinterpretation of previous studies involving the 39K protein-immunoprecipitating mabs. Of primary importance is the fact that the demonstrated immunoprotective ability of a 39K protein-immunoprecipitating mab (E. J. Hansen, S. M. Robertson, P. A. Gulig, C. F. Frisch, and E. J. Haanes, Lancet i:366-368, 1982) must now be regarded as evidence that antibody directed against Hib LPS can be protective against experimental Hib disease.