The activity of erythrocyte membrane Na-pumping K~+-activated pera-nitro-phenylphosphatase (K~+-pNPPase) and red cell deformability (RCD) were stndied in 50adult patients with non-insulin-dependent diabetes mellitus (NIDDM). RCD was expressedas red blood cell filtration index (RBC-IF). In comparison with the healthy subject, theactivity of erythrocyte membrane Na-pumping K~+-pNPPase was significantly decreased,and RBC-IF was significantly increased. The above-mentioned changes were more severein NIDDM with retinopathy than those without. There was a very close negative corre-lation between Na-pumping K~+-pNPPase and RBC-IF. These results indicated that thedecrease of erythrocyte membrane Na-pumping K~+-pNPPase activity might be an importantfactor fot the decrease of RCD.