AbstractAn unexpected finding concerning our previously reported polyclonal antibody raised against an extract from human amnion (pAb 6/2, also termed AA3), and which recognizes an epidermal keratinocyte protein, is presented in this study. Using the immunoblot technique, pAb 6/2 binds to a 37 kD intracellular protein antigen. We have subsequently found that, by radioimmunoprecipitation performed after metabolic labelling with35S‐methionine of cultured keratinocyles, pAb 6/2 recognizes the 600 kD epidermal basement membrane component (termed BM‐600/nicein) which was reported to be bound by the monoclonal antibody mAb GB3. Specifically, pAb 6/2 reacts with immunoaffinity chromatography‐isolated BM‐600/nicein blotted onto nitrocellulose. The data suggest the existence of two immunological reactivities borne by pAb 6/2, each of them being directed against, respectively, the 37 kD (seen in immunoblots) and the 600 kD protein (seen in immunoprecipitations). The data further suggest possible independent expression of these two proteins in cell culture. In comparison with the staining pattern of normal skin, immunofluorescence was previously noted to be impaired (pAb 6/2) or absent (mAb GB3) in lethal junctional epidermolysis bullosa. Thus, we conclude that mAb GB3, rather than pAb 6/2, is a more appropriate probe for the comprehensive biochemical study of this genoder