Enzyme-linked immunosorbent assay (ELISA) was applied to the detection of IgG and IgM antibodies against outer membrane vesicles (OMV) antigen of Haemophilus influenzae type b. In this ELISA system, IgG antibody titers were about 40 fold higher than those in indirect hemagglutination assay (IHA). The IgG antibody titers by this ELISA of rabbit sera obtained after immunization were comparable with those by radioimmunoassay (RIA) of the same sera. A significant correlation was established between these two assays (r = 0.973, P less than 0.001).