Two automatic apparatuses utilizing a dual wave-length spectrophotometric method were developed to perform oxygen dissociation curves on microsamples of blood. This method provides a complete printout of an oxygen dissociation curve in 15 to 20 minutes and the P50 values obtained agree closely with those obtained by classical methods. This technique was used as part of a routine clinical evaluation in assessing oxygen delivery in anemic patients. These apparatuses allowed identification of patients with well-compensated anemia, patients with no compensatory decrease in oxygen affinity, and several patients with a paradoxical increase in oxygen affinity. Knowledge of the oxygen-releasing capacity of the blood permits a more thorough evaluation of oxygen transport in anemic patients than the hemoglobin level and a more precise assessment of the need for transfusion therapy. The ready availability of an oxygen dissociation curve by this method is feasible for routine clinical evaluation and permits more judicious use of blood products.