The effect of hemoglobin polymerization on the facilitated transport of oxygen in systems containing polymerized hemoglobins has been studied experimentally. In part I of this work, the oxygen transport efficacy of a candidate blood substitute based on polymerized bovine hemoglobin was studied. The diffusion of dissolved oxygen in the polymerized hemoglobin samples was not different from that in the unaltered hemoglobin solutions. However, in the high oxygen tension gradient (HOTG) decreasing pO\sb2 experiments at 37\sp∘C, while the augmented oxygen transport was almost double than that of the simple diffusive flux of oxygen in the case of the latter, it was only increased by 30% in the case of polymerized hemoglobin. The lower facilitated oxygen transport is due to the decreased diffusion coefficient as well as the decreased oxygen affinity of the polymerized hemoglobin. For lower pO\sb2 gradients in the range of physiological significance (constant, low oxygen tension gradient, CLOTG, experiments), the oxygen transport in the polymerized hemoglobin samples was approximately the same as in unpolymerized sample over a wide range of oxygen tensions. At lower oxygen tensions, there was a significant augmentation effect, less than in the HOTG experiment and less for the polymerized than for the unploymerized Hb. In order to further confirm the results of diffusion cell experiments, in part II of this work, the diffusion coefficients of various unpolymerized and polymerized hemoglobins were measured using the PFG NMR technique. The diffusion coefficient values were in good agreement with those estimated from the diffusion cell experiments. The effect of hemoglobin polymerization on oxygen transport in sickle erythrocytes at 37\sp∘C was studied in part III of this work. HOTG and CLOTG decreasing pO\sb2 diffusion cell experiments conducted on packed sickle cells show negligible enhancement of oxygen transport by sickle hemoglobin. Diffusion coefficients of unpolymerized HbS in the solution phase of slowly deoxygenating sickle cells was measured using PFG NMR technique. After an initial decrease the hemoglobin diffusion coefficient stays constant suggesting that though the polymer does offer appreciable resistance, it is not very high so as to drastically reduce the diffusion of hemoglobin and oxygen.