A procedure was developed which renders possible the identification of water-insoluble membrane proteins by immunoelectrophoresis in a solubilizing solvent. Purified erythrocyte membranes from the pig were extracted with butanol, and the protein components were solubilized in a Tris-HCl buffer containing urea and Triton X-100 at reduced concentrations throughout the procedure. After removal of 12% insoluble protein by centrifugation, 50% of the remaining proteins appeared to be monomers and 50% polymers, i.e. tetramers. Immunoelectrophoresis in the above buffer mixtures resulted in six precipitation lines. The specificity of the reaction was established. It seems that the multiple-site interactions of antigen and antibody have sufficient energy to allow some reduction by addition of the solubilizing agents, urea and Triton X-100, without losing specificity.