Chemical modifications of human or bovine hemoglobins are designed to produce proteins that can act as oxygen-carrying blood substitutes. Concerns about the redox reactivity of cell-free hemoglobin and its contribution to tissue-damaging oxygen free radicals has not been fully established. We determined that bovine hemoglobins intra- or intermolecularly crosslinked differ in their ability to generate or interact with reactive oxygen species. These differences do not correlate with their oxygen affinities. We compared HbBv-FMDA, produced by the reaction of bovine hemoglobin with fumaryl-monodiaspirin and Poly HbBv, a glutaraldehyde polymerized bovine hemoglobin, with unmodified bovine hemoglobin (HbBv). Superoxide radicals are produced during the spontaneous oxidation of hemoglobin. Relative to the other two proteins. Poly HbBv was found to be more susceptible to autoxidation. Spectral changes indicative of protein modification and ferrylhemoglobin formation during the enzymatic peroxidation of these hemoglobins differ qualitatively and occur at an increasing order, poly HbBv > HbBv > HbBv-FMDA. The proteins also differ in the rate of hemoglobin catalyzed NADPH oxidation and aniline hydroxylation, reactions mediated by reactive oxygen species. Taken together, our results and those reported previously on modified human hemoglobins, suggest that redox and oxygen-carrying functions of hemoglobin can be experimentally manipulated as independently selectable parameters that may ultimately aid in the design of a safer reperfusion agent.