Insulin degradation by human erythrocyte fractions was studied using the TCA-precipitation method. Hemolysate exhibited an insulin degrading activity higher than membranes. Triton X-100 treatment of membranes led to the appraisal of Triton-soluble degrading activity and of a more efficient Triton-not-soluble degrading activity. Monovalent cations (Na+, K+, Li+) did not modify the insulin degradation by any of the erythrocyte fractions. Divalent cations, Ca++ and Zn++ selectively enhanced insulin degradation by the membranous fractions, and Cu++ and Zn++ strongly inhibited insulin degradation by all the erythrocyte fractions. The results supported the hypothesis of the existence of at least two different degrading systems in human erythrocytes: soluble (cytosolic) Ca++ and Mg++ insensitive system(s) and membrane associated Ca++ and Mg++ sensitive system(s).