OBJECTIVE: C1 inhibitor (C1-INH) is an endogenous inhibitor of complement and kinin systems. We have explored the efficacy and the therapeutic window of the recently available human recombinant (rh) C1-INH on ischemic brain injury and investigated its mechanism of action in comparison with that of plasma-derived (pd) C1-INH. METHODS: rhC1-INH was administered intravenously to C57Bl/6 mice undergoing transient or permanent ischemia, and its protective effects were evaluated by measuring infarct volume and neurodegeneration. The binding profiles of rhC1-INH and pdC1-INH were assessed in vitro using surface plasmon resonance. Their localization in the ischemic brain tissue was determined by immunohistochemistry and confocal analysis. The functional consequences of rhC1-INH and pdC1-INH administration on complement activation were analyzed by enzyme-linked immunosorbent assay on plasma samples. RESULTS: rhC1-INH markedly reduced cerebral damage when administered up to 18 hours after transient ischemia and up to 6 hours after permanent ischemia, thus showing a surprisingly wide therapeutic window. In vitro rhC1-INH bound mannose-binding lectin (MBL), a key protein in the lectin complement pathway, with high affinity, whereas pdC1-INH, which has a different glycosylation pattern, did not. In the ischemic brain, rhC1-INH was confined to cerebral vessels, where it colocalized with MBL, whereas pdC1-INH diffused into the brain parenchyma. In addition, rhC1-INH was more active than pdC1-INH in inhibiting MBL-induced complement activation. INTERPRETATION: rhC1-INH showed a surprisingly wider time window of efficacy compared with the corresponding plasmatic protein. We propose that the superiority of rhC1-INH is due to its selective binding to MBL, which emerged as a novel target for stroke treatment.