Four solutions of bovine polymerized hemoglobin (BPHS) and rabbit plasma were used to replace one-third of the blood volume in five groups of rabbits. The first three solutions were "impure" because of the presence of stromal phosphatidyl-ethanolamine and phosphatidyl-serine in BPHS-1, environmental endotoxins in BPHS-2, and a large amount of higher molecular weight hemoglobin-glutaraldehyde polymers in BPHS-3. These solutions caused a 33 per cent mortality rate and significant morbidity which was characterized by hemodynamic instability, respiratory and renal insufficiency, elevation of hepatic enzyme levels, thrombocytopenia, leukopenia, disseminated intravascular coagulation (DIC) and activation of the alternate pathway of complement. Histopathologic changes found in the heart, lungs, liver, spleen and kidney were characterized by a combination of ischemic and inflammatory lesions. Fibrin thrombi were visible by immunofluorescence in the microcirculation. In contrast, the fourth solution (BPHS-4) was free of the aforementioned impurities; caused no deaths and minimal morbidity, which was limited to elevated levels of serum glutamic pyruvic transaminase and reduction of creatinine clearance; no DIC or complement activation, and mild histopathologic changes which were exclusively ischemic in nature. The results of this study indicated that the toxicity of polymerized hemoglobin solutions is due principally to the presence of impurities. Pure hemoglobin does exhibit mild toxicity when compared with a control solution which is most likely due to a vasoconstrictor effect of oxyhemoglobin.