Three monoclonal antibody (MAb)-based immunoassays were developed for specific detection of Haemophilus influenzae type b (Hib) lipooligosaccharide (LOS). (i) Hib LOS was captured onto microtiter plates by polyclonal Hib-directed antibodies and detected with MAbs to the oligosaccharide component of Hib LOS in an enzyme-linked immunosorbent assay, (ii) The high affinity of polymyxin B for lipid A was used to bind Hib LOS to microtiter wells, and the oligosaccharide-specific MAbs were used as the detection system in the polymyxin B-MAb assay. (iii) Hib LOS solubilized in detergent was captured by MAbs, and the immobilized LOS was detected with a chromogenic Limulus amebocyte lysate method in the immunolimulus assay. Endotoxin concentrations were measured in in vitro samples and cerebrospinal fluid samples from rabbits with experimental Hib meningitis. The results were compared with those obtained with the standard chromogenic Limulus amebocyte lysate assay. There were significant correlations between the results of all four assays. These new immunoassays provide methods for specific detection of Hib LOS in laboratory fluids and in research involving quantification of Hib endotoxin in experimental animal models.