Background and Purpose- A hemoglobin-albumin cluster, 1 core of hemoglobin covalently bound with 3 shell albumins, designated as HemoAct was developed as a hemoglobin-based oxygen carrier. We aim to investigate neuroprotection by HemoAct in transient cerebral ischemia and elucidate its underlying mechanisms. Methods- Male rats were subjected to 2-hour transient middle cerebral artery occlusion and were then administered HemoAct transarterially at the onset of reperfusion. Neurological and pathological findings were examined after 24 hours of reperfusion to identify neuroprotection by HemoAct. Intermittent measurements of cortical blood flow and oxygen content were performed, and a histopathologic analysis was conducted on rats during the early phase of reperfusion to assess the therapeutic mechanism of HemoAct. In addition, the antioxidant effects of HemoAct were examined in hypoxia/reoxygenation-treated rat brain microvascular endothelial cells. Results- Neurological deterioration, infarct and edema development, and the activation of MMP-9 (matrix metalloprotease-9) and lipid peroxidation after 24 hours of reperfusion were significantly ameliorated by the HemoAct treatment. Reductions in blood flow and tissue partial oxygen pressure in the cortical penumbra after 6 hours of reperfusion were significantly ameliorated by the HemoAct treatment. The histopathologic analysis of the cortical penumbra revealed that HemoAct in HemoAct-treated rats showed superior microvascular perfusion with the mitigation of microvascular narrowing changes than autologous erythrocytes in nontreated rats. Although HemoAct extravasated into the ischemic core with serum protein, it did not induce an increase in serum extravasation or reactive oxygen species production in the ischemic core. In vitro experiments with rat brain microvascular endothelial cells revealed that HemoAct significantly suppressed cellular reactive oxygen species production in hypoxia/reoxygenation-treated cells, similar to albumin. Conclusions- HemoAct exerted robust neuroprotection in transient cerebral ischemia. Superior microvascular perfusion with an oxygen delivery capability and possible antioxidant effects appear to be the underlying neuroprotective mechanisms.