Rheological changes in the blood flow and in the blood cell themselves may contribute to the appearance and evolution of diabetic retinopathy. In relation to the sludging phenomenon of erythrocyte in diabetes mellitus, surface electric charge (SEC) and membrane N-acetylneuraminic acid (NANA) were measured. The SEC and NANA levels of erythrocyte were determined by the colloid titration and by Warren's thiobarbituric acid method, respectively. The SEC levels of erythrocyte were significantly lower in diabetics without retinopathy (group A)(5.94±0.39×1017/ml packed cell, N=25)(P<0.01) than in controls (6.38±0.45, N=15) and in diabetics with retinopathy (group B)(5.70±0.27, N=17)(P<0.05) than in group A. The membrane NANA levels of erythrocyte were also significantly lower in group A (0.32±0.10 ttmol/m/ packed cell, N=25)(P<0.05) than in controls (0.39±0.09, N=15) and in group B (0.28±0.05, N=17)(P<0.05) than in group A. It may be conceivable that the above results contribute to the solution of rheological changes of blood in diabetes mellitus.