A dual track approach has been applied to the study of oxygen transport behavior of erythrocyte/hemoglobin mixtures flowing in microvessels. The method includes experimental and theoretical modeling of the microcirculation. An experimental in vitro 27 μm diameter capillary model was developed to provide detailed oxygen flux measurements for homeglobin solutions, erythrocyte suspensions, and erythrocyte/hemoglobin solution mixtures. The experimental apparatus includes computerized data acquisiton and control coupled to a Leitz Toolmaker Microscope-based, dual wavelength microspectrophotomer. Fractional oxygen saturation may be determined for various axial positions, and the resulting experimental data have been shown to agree well with simulations calculated from previously developed theoretical models of oxygen transport in hemoglobin solutions and rbc suspensions. Direct comparison of hemoglobin solutions with rbc suspensions of the same overall hemoglobin concentration and oxygen affinity shows that hemoglobin solutions are more efficient transporters of oxygen. Experiments on polymerized hemoglobin suggests that some heme pockets may have not be available for oxygen transport. The quality and quantity of the experimental data represent a significant improvement over previous experimental designs, and the results confirm existing oxygen transport models. Ultrapurified, native and polymerized bovine hemoglobin/red blood cell mixtures were studied in the capillary. Dose response plots were generated by varying the extracellular to intracellular hemoglobin distribution ratio. Increased extracellular hemoglobin concentration increased oxygen transport efficiency for both uptake and release. When only 10% of the total hemoglobin was extracellular, half of the increased efficiency of pure hemoglobin solutions was reached. When half the hemoglobin was extracellular, the mixtures behaved like hemoglobin solutions. A mathematical model of the mixture experiments initially failed to fully predict the enhancement of oxygen uptake, but well described release experiments. Consideration of shear induced augmentation led to a new hypothesis for the relative importance of extracellular mixing in rbc/hemoglobin mixtures flowing in arteriolar sized conduits. The resulting semi-predictive model matches experimental data for a variety of conditions. This model may be used to predict performance of hemoglobin-based blood substitutes.