Stroma-free hemoglobin solutions have been the subject of extensive studies as potential acellular oxygen carriers. Oncotic pressure considerations limit the hemoglobin concentration of the solutions (6-8 g/dl) to one-half the normal whole blood values. Furthermore, the free hemoglobin has a short plasma half-life (2-4 hours). In principle, polymerization offers a means of normalizing the oxygen-carrying capacity as well as extending the plasma half-life. Pyridoxylation of hemoglobin prior to polymerization provides an acceptable P50. Pyridoxylated hemoglobin (14-16 g/dl) was polymerized with a 12.5 percent glutaraldehyde solution. Since the goal was to obtain a 15 g per dl solution iso-oncotic with plasma, the polymerization reaction was monitored by the drop in colloid osmotic pressure. The reaction was quenched with 1.3 M lysine when the colloid osmotic pressure reached normal values (20-25 torr). The polymerization yield was 80 percent, with molecular weights ranging from 120,000 to 600,000 Dalton. The polymerized hemoglobin had a binding coefficient of 1.32, a P50 of 16 torr, a Bohr coefficient of -0.12, and a Hill coefficient of 1.7. The viscosity of the solution was 4.5 centipoise. The methemoglobin levels were comparable to that of unpolymerized stroma-free hemoglobin. Polymerized hemoglobin solutions provide a normal oxygen-carrying capacity with a P50 comparable to that of unpolymerized stroma-free hemoglobin solutions.