Erythrocyte membrane fluidity was studied by means of electron spin resonance in 15 uremic, hemodialyzed patients and 14 normal subjects. Erythrocyte membrane fluidity determined using a 16-nitroxide stearic acid spin label probe was of a significantly lower level in the uremic patients, when compared with normal control subjects. Alterations in molar ratios of membrane free cholesterol to phospholipid are probably not a principal factor contributing to this change in fluidity. Significant decreases of phosphatidylcholine and molar ratios of phosphatidylcholine to sphingomyelin were noted in the erythrocyte membrane of uremic patients, and these alterations may relate to the fluidity change.