Antigenic reactivities of 4 purified CEA preparations and 4 different CEA-related normal antigens (NCA from lungs, NCA-2 from meconium, and NFA-1 and NFA-2 from adult feces) were comparatively analyzed with seven commercially available EIA kits [Abbott CEA-EIA Monoclonal, D-ZYME CEA, Imzyne CEA, CEA MITSUI II, CEA Roche EIA, Immunoball-CEA (N), and Glaozyme CEA]. All kits employ a sandwich-type solid-phase method using polystyrene beads and monoclonal anti-CEA antibodies as either capture antibody and/or tracer antibody. In general, all CEAs reacted well with these assay kits. Reactivity differences in weight among the CEA preparations were, however, observed in all but one assay kit (D-ZYME CEA) in which all CEAs showed fairly homogeneous reactivity. The degree of reaction intensity among the preparations used varied depending on the assay kits used. NCA, which is well known to be partially cross-reactive with CEA, revealed a negligible reactivity in all kits. NFA-1 which is also partially cross-reactive with CEA but antigenically unrelated to NCA, showed very strong reactivity in 2 kits [CEA Roche EIA and Immunoball-CEA (N)] and weak reactivity in another kit (CEA MITSUI II). NCA-2 in meconium and NFA-2 in normal adult feces, which both have immunochemical and physicochemical properties very similar to those of CEA, reacted to a greater or lesser extent with all kits. However, one assay kit (D-ZYME CEA), whose reactivity to different CEAs was fairly homogeneous, could discriminate CEAs from NCA-2 and NFA-2. Although the reactivity of NFA-2 was less than that of any CEA in all kits, that of NCA-2 was higher than that of some CEAs in 3 kits (Abbott CEA-EIA Monoclonal, CEA MITSUI II, and CEA Roche EIA). These differences in reactivity and specificity of currently available EIA kits for CEA should be borne in mind when selecting an assay kit.