We have compared the demethylation rate of protein carboxyl methyl esters from isolated human erythrocyte membranes with the corresponding rate of metabolic turnover of these same methyl groups in the intact erythrocyte. Surprisingly, the apparent spontaneous demethylation of these membrane protein methyl esters was significantly faster at physiological pH than the corresponding rate determined by pulse-chase analysis of intact cells incubated with L-[methyl-3H]methionine. Readdition of erythrocyte lysate to purified membranes did not increase the rate of demethylation, as might be expected if there were cytosolic or membrane-bound protein methylesterase activity, but resulted instead in an apparent stabilization of these methyl esters. Thus, the metabolic lability of these protein methyl esters in intact cells may be quantitatively explained by spontaneous, rather than enzymatic, demethylation reactions. A model is presented in which a rapid but nonenzymatic intramolecular demethylation reaction results in the formation of a polypeptide imide or anhydride intermediate. The metabolic fate of these hypothetical intermediates is unknown but may lead to the repair or degradation of protein D-aspartyl and L-isoaspartyl residues, which appear to be the substrates for the initial transmethylation reaction.