One or several short episodes of non-lethal ischemic episodes administered to a heart prior (Ischemic Preconditioning, IPC) or directly after (Ischemic Postconditioning, IschPost) a major ischemic episode facilitates myocardic survival during the reperfusion phase. There is strong evidence that this protection is due to a factor released from the heart during the conditioning phase. The perfusate containing protects the heart when administered 10 minutes prior and after ischemia (253% and 303% vs. 553% for control, p < 0.05). The factor is so potent that when administered only in 3 x 30" bursts after ischemia it is enough to significantly lower infarct sizes (405% vs. control 564%, p < 0.05. We have attempted to characterise and isolate this factor from effluate released during preconditioning of isolated rat hearts in a Langendorff setup. We fractionated the proteins in the effluent based on both size and hydrophobicity and the fractions were administered to unconditioned rat hearts prior to a 30 min ischemic event and the infarct/risk ratio was calculated after 120 min reperfusion. Hearts perfused with a hydrophobic fraction over 10 kDa had ischemic injuries at the level of an IPC heart (256% vs. 223%, p < 0.05) and over two times lesser injuries than the hearts that were perfused with a hydrophilic fraction and the hearts that were perfused solely with Krebs Heinsleitt Buffer (KHB) (253% vs. 553% and 5012%, p < 0.05). We analysed the different fractions by LC- MSMS and identified a total of 88 proteins released during preconditioning and of several of them were only identified in fractions which were protective.