The biophysical properties of red blood cells in three groups of chronic renal failure (CRF) patients were compared to those of a non-renal control group. A limited number of biochemical and other parameters influencing the physical characteristics of red cells were also studied. Physical characteristics varied from normal to strikingly abnormal depending on the presence of factors other than CRF. Uraemia per se, in contrast to general belief, would appear not to be the major determinant of red cell survival in CRF.