The spectrophotometric determination of hemoglobin-oxygen dissociation on dilute red cell suspensions has been modified by the use of a bicarbonate buffer containing bovine serum albumin. The red cell suspension is equillibrated with known ratios of air and nitrogen and 5 per cent carbon dioxide at atmospheric pressure using two gas pumps. The method enables the hemoglobin-oxygen dissociation curve to be measured on 10 to 20 mul of whole blood, and has given values for the P50 (the oxygen tension at 50 per cent hemoglobin-oxygen saturation) of 25.9 plus or minus 1.5 S. D. which are comparable to the value of 26.0 plus or minus 1.0 S. D. obtained by the mixing technique and by other methods. The addition of bovine albumin (35 mg. per cent or greater) increased the P50 by 3 mm. Hg above that obtained with buffer alone. The P50 value has been compared in bis-Tris and phosphate buffers with and without albumin, and in bicarbonate buffers with the addition of approximately equimolar concentrations of either human or bovine serum albumin or polyvinyl-pyrrolidone (PVP), and buffer alone. The effect of albumin on the P50 value was most marked in bicarbonate buffer, and was statistically significantly greater in the presence of human or bovine albumin than in buffer alone or buffer plus PVP. The addition of albumin could not be explained through an alteration of cell shape or volume, but may be an influence on intracellular pH.