Abstract: Efforts to develop alternatives to blood as an oxygen carrier date back well over half a century with increased efforts in recent years being partly motivated by fears of potential contamination with the viruses responsible for AIDS and hepatitis (1-4). An additional impetus rests in the recognition that wide availability of properly banked blood is absent in many parts of the world. An efficient acellular oxygen carrier could have several distinct advantages over intact red blood cells, including elimination of the need for typing and cross-matching before transfusion and a useful shelf-life greater than the current six weeks or less for packed red blood cells (1). While a number of other alternatives have been explored over the years (1-4), the use of cell-free hemoglobins for an oxygen-carrying resuscitation fluid has excellent prospects since (a) hemoglobin solutions are completely metabolizable and are well tolerated by the body, and (b) hemoglobin is available in virtually unlimited amounts and is relatively inexpensive, (c) hemoglobin is fully saturated with oxygen under ambient conditions, has oncotic activity, and exhibits cooperative oxygen binding behavior. However, there are two major problems associated with using cell-free hemoglobins for transfusions. First, the retention time of cell-free hemoglobins in circulation after infusion is very short (4), and most of the infused hemoglobin is rapidly filtered and eliminated by kidneys in 1-4 hours; second, cell-free hemoglobins have too high oxygen affinity that prevents them from adequately unloading the oxygen acquired from lungs to tissues. Both of these problems have their roots in 2,3-diphosphoglycerate (DPG) and other polyanionic species which are conspicuously absent in cell-free hemoglobins, but are known to be essential co-factors in intact red blood cells. Efforts have since been focused on covalently cross-linking hemoglobin between the two like subunits, e.g., alpha-1 to alpha-2 or beta-1 to beta-2. Such a cross-link is believed to substitute for the native allosteric modifier DPG to lower the oxygen affinity, while at the same time preventing dissociation of the tetramer (1).