The use of recombinant hemoglobin as an oxygen carrier in artificial blood substitutes has had limited success due to the toxicity of the free molecule. Now a team of researchers has produced a recombinant hemoglobin that appears to solve many of these problems and may eventually form the basis for a novel class of artificial blood products. Recombinant hemoglobin in a blood substitute needs to have a lower affinity for oxygen than natural hemoglobin, as it has to function in the absence of effector molecules found inside red blood cells. Unfortunately, hemoglobin variants with low oxygen affinity tend to be less stable, and recombinant hemoglobin also tends to produce hypertension by interfering with the nitric oxide signaling pathway. In the new work ( Biochemistry 39, in press, 2000), senior author Chien Ho and his colleagues identified two amino acid changes that lead to low oxygen affinity, high stability, and resistance to nitric oxide-induced oxidation in hemoglobin. “[The molecule] shows great potential as a successful oxygen carrier, and is something that could realistically be used in people one day,” Before it reaches humans, the new hemoglobin will need to be produced on a larger scale, possibly in transgenic animals.