The formation of biologically active hemoglobin fragments in human erythrocytes was studied. The structures of 33 peptide products of intraerythrocytic hemoglobin cleavage were determined. Based on an analysis of these sequences, a model of the stepwise degradation of the hemoglobin alpha- and beta-chains was suggested. The processes of peptide formation in a cell-free erythrocyte lysate system were studied. The involvement of an enzymatic complex of the cell membrane fraction was demonstrated. It was found that the cells of a surviving human erythrocyte culture secrete short (of 5-20 amino acid residues) peptides, and the structures of 36 peptides were determined. The dynamics of peptide secretion was investigated, and preliminary data on the energy-dependence of this process were obtained. Based on the experimental results, a model describing erythrocytes as an endocrine gland was suggested.