Ionic liquid was for the first time employed for selective isolation of heme-protein species. Direct extraction of hemoglobin into ionic liquid without using any concomitant reagent or extractant was carried out. Hemoglobin at the level of 100 ng microL(-1) could readily be quantitatively extracted into ionic liquid (IL) 1-butyl-3-trimethylsilylimidazolium hexafluorophosphate (BtmsimPF(6)) in the absence of any co-existing extractants/additives at pH 7, at the same time; however, the other protein species do not interfere and remain in the aqueous phase. A back extraction efficiency of ca. 80% for 20 ng microL(-1) hemoglobin in ionic liquid phase was achieved with sodium dodecyl sulfate (SDS) solution as stripping reagent. (57)Fe Mossbauer spectra and circular dichroism (CD) spectra indicated that the penta-coordinated ferrous atom in hemoglobin provide a vacant or free coordinating position, which could be occupied by the cationic Btmsim(+) moiety. The interaction/coordination reaction between the iron atom in the heme group of hemoglobin and the cationic ionic liquid moiety furnishes the driving force for facilitating fast transfer of hemoglobin into BtmsimPF(6). The present system was applied for selective isolation of heme-protein, i.e., hemoglobin from human whole blood without any pretreatment, giving rise to satisfactory results.