Abstract This paper presents studies on the characterization of the protein component of mammalian erythrocyte membranes. Exhaustive lipid extraction of the membrane results in a glycoprotein residue containing greater than 90% of the total membrane protein. The composition of this membrane preparation is presented. Several methods have been developed to solubilize this protein. The protein is soluble in formic acid and sodium dodecyl sulfate solutions. After total succinylation of the protein, complete solubility is achieved in aqueous solutions at neutral pH. A new method for completely solubilizing the unmodified membrane protein in urea and guanidine hydrochloride solutions at neutral pH in the absence of detergent is presented. This method depends on initial solubilization of the membrane protein in phenol. End group analysis, electrophoresis, ultracentrifugation, and chromatography in these solvent systems reveal the protein in membranes to be a heterogeneous collection of proteins, many in the molecular weight range near 50,000. The membrane preparation and the solvent systems described will be valuable in the characterization of the individual protein molecules present in the membrane. These studies are now in progress.