Stress proteins are assumed to protect cells against various kinds of stresses including ischemia. In this study, we focused on the behaviour of the most abundant myocardial stress protein,αB-crystallin, during ischemia and reperfusion of the pig heartin vivo.αB-crystallin constitutes 1–2% of the soluble protein pool and underwent, during severe but reversibly damaging ischemia (25 min), complete translocation to the Z-line area of myofibrils. Irreversibly damaging ischemia (60 min) was accompanied by extreme stretching of the majority of myofibrils, and by concomitant extension ofαB-crystallin localization from the Z-line area to I-bands. This I-band shift correlated with displacement of the T₁₂epitope of titin from the vicinity of Z-lines into I-bands, indicating that the primary binding sites forαB-crystallin might also be located in juxtaposition to Z-lines and move into the I-bands during extreme sarcomeric stretching. During reperfusion after 25 min of ischemia,αB-crystallin disappeared rapidly from myofibrils; whereas reperfusion after irreversibly damaging ischemia (60 min) resulted in dissociation ofαB-crystallin only from those myofibrils and myocardiocytes that were still able to contract, andαB-crystallin remained bound to the overstretched, damaged myofibrils no longer capable of contraction. The time course of translocation ofαB-crystallin to myofibrils during ischemia correlated with phosphorylation of approximately 20% of the entireαB-crystallin pool. However, disappearance ofαB-crystallin from myofibrils during reperfusion was not accompanied by dephosphorylation, indicating that phosphorylation alone does not explain myofibrillar binding ofαB-crystallin. Ischemia-induced myofibrillar targeting ofαB-crystallin probably requires additional structural and posttranslational modifications of myofibrillar components in juxtaposition to I-bands.