Enzyme linked immunosorbent assay (ELISAs) has been used as a convenient routine immunological assay for many specific antigens to monitor antibody responses to vaccination, and has replaced other methods, e.g., radioimmunoassay (FARR assay) (1,2). For example, ELISA using tetanus toxoid, 1diphtheria toxoid, Haemophilus influenzae Type b (Hib) polysaccharide and Neisseria meningitidis A/C polysaccharide are used to determine serum antibodies following vaccination. In these assays, the antigen, e.g., toxoid (tetanus, diphtheria) or polysaccharide (Hib PRP, Men C) alone or conjugated to poly-L-lysine or methylated human serum albumin (mHSA), is coated onto plastic of 96-well microtiter plates (3-5). By direct or indirect methods the antibodies in serum samples can be detected using enzyme-linked secondary antibody conjugates, e.g., horseradish peroxidase or alkaline phosphatase, and appropriate substrates using color reaction at specific absorbance e.g., ODA_405. The principle of the indirect ELISA is shown in Fig. 1. (6). Using dilutions of a standard serum or monoclonal antibody with known concentration of total/ specific antibody or given arbitrary units, it is possible to compare dilutions of unknown samples against a standard curve. In this chapter we demonstrate how to establish an ELISA for antibody to H. influenzae (Hi) inner-core lipopolysaccharide (LPS) epitopes.