The development and evaluation of an effective Hb solution as a blood substitute are important not only for the care of casualties resulting from mass disasters, but also for eventual use in other special clinical situations. Substantial improvements have been made in recent years in the quality of Hb solutions. Solutions of unmodified Hb, although with certain limitations as indicated, potentially could be useful in several applications. The limitations presented by unmodified Hb can be overcome by a modification of the Hb molecule. In oxygen transport, not only the flow of the vascular fluid but also the vascular retention time, the oxygen affinity, and the concentration of free circulating Hb are important. A solution of Pyr-Poly Hb possesses positive characteristics in regard to the last three of these factors and, if the possibility of using bovine Pyr-Poly Hb is considered, the problem of supply for the material needed for the stockpiling of large quantities of this resuscitating solution can be eliminated. Products obtained by such modification must be evaluated also for their safety so that no potential adverse clinical effects would develop when administered to human patients.