Stroma-free homoglobin solution has received extensive experimental trials in dogs and nonhuman primates. It has been shown to have no coagulant activity and to have no deleterious effect on renal function. Also pathological examination of autopsy and biopsy specimens fails to demonstrate any kidney damage after infusion of large amounts of stroma-free hemoglobin solution. In vivo data would also indicate that the solution can both carry oxygen and liberate it into tissues despite the fact that its in vitro oxygen dissociation curve is shifted to the left. Animals can be maintained with a 3% hematocrit after exchange transfusion with hemoglobin solution, without any significant fall in blood pressure or rise in central venous pressure. This is not true for nonoxygen carrying plasma expanders. Hemoglobin solution has also been shown to have an effect on blood rheology which would tend to increase flow through microcirculation. We conclude that stroma-free hemoglobin solution has potential for use as a plasma expander for treatment of hemorrhagic and possibly other types of shocks, as well as other uses.