We have recently described a monoclonal antibody (X222) which localizes at pore complexes in Xenopus oocytes and immunoblots a protein under nonreducing conditions with an apparent molecular mass of 215 kDa (Cordes, Gajewski, Stumpp, Krohne, Differentiation 58, 307-312, (1995)). Under reducing conditions, this antigen decreased from the previously reported 215 kDa to 200 kDa. Since this protein binds to the lectin concanavalin A (Con A), we now refer to it as gp200. In addition to Con A binding, Xenopus gp200 has several other features in common with the mammalian pore membrane protein gp210. On nuclear envelopes extracted with Triton X-100, gp200 was localized on the lateral walls of pores, a subdomain covered in native nuclear envelopes by the lipid bilayer. In mitotic extracts of Xenopus eggs gp200 was exclusively recovered in the membrane fraction. Treatment of oocyte membranes with urea and proteases indicated that gp200 is an integral membrane protein with the bulk of its mass located within the membrane lumen. In Xenopus oocyte nuclei gp200 is the major Con A binding protein. The new monoclonal antibody X222 has allowed us to characterize the first integral membrane protein, gp200, of the Xenopus pore complex.