A novel approach to the study of erythrocytes instability based on assessment fusability of their plasma membrane is presented. The fusability of erythrocytes normal blood, stored for 7 days at 4°C, and type 2 patients with diabetes were studied. Human erythrocytes were fused by incubation with La³⁺ at final concentrations from 50 to 200 μM. Erythrocytes aggregates were incubated at 37°C for 120 min. The fusability of erythrocytes was evaluated using a light microscope. It was shown that La³⁺ induced extensive fusion of erythrocytes after 7 days of blood storage and erythrocytes of patients with diabetes. Incubation of normal erythrocytes at 37°C for 120 min does not induce cell fusion.