Damage to the internal viscosity of red cell is correlated with changes in the surface area to volume ratio and to prelytic phenomena. Considerable changes in shape were noticed in red cells from shocked dogs and some critically ill patients in blood smear or fresh preparation, or after in vitro treatment with colloidal plasma substitutes or glucose, and so erythrocyte filterability was investigated in critically ill patients, given or not given parenteral nutrition. The results revealed an increase of the erythrocyte filtration time (EFT 1/2) in more than 50% of the critically ill patients and less alteration in critically ill patients given total parenteral nutrition, the mortality being half as great. The return of increased EFT 1/2 to normal values occurred after 3-12 days of total parenteral nutrition in 6 patients investigated. A death rate twice as great was seen in patients with high EFT 1/2 as compared to the 27.7% mortality in patients with normal EFT 1/2, its correlation with increased filtration time being significant, r = 0.81 greater than 3 Sr, y = 1.291x + 27.44, n = 47.