Studies will be continued of effects of loss of red cell deformability upon transit of red cells through a variety of vascular beds. Two factors influencing red cell deformability are under investigation: the presence of abnormal hemoglobin within red cells and exposure of red cells to hypertonic solutions. In a typical study, normal human red cells are labeled with one isotope and mixed with abnormal human red cells (e.g., sickle cells or treated sickle cells) and these are injected into the artery of an organ (e.g., the lung of an anesthetized dog). Serial samples of venous blood are collected for seconds and the relative transit times of these red cell populations are compared. In other studies, small injections of very hypertonic solutions are introduced into the artery and hemoglobin and plasma protein concentrations in the collected blood are determined. Excess hemoglobin retention within the organ is calculated and related to loss of red cell deformability. The quantity of tissue fluid extracted by osmotic gradients is determined and capillary reflection coefficients of a variety of solutes may be compared. Solute concentrations in the extracted tissue fluid are calculated from changes in whole blood solute concentrations.