The ability of ceruloplasmin, an important serum antioxidant, to reduce the vulnerability of the isolated rat heart to reperfusion arrhythmias has been investigated. Bovine plasma ceruloplasmin was purified by chromatography on aminoethyl-agarose. Isolated rat hearts were submitted to 15 min of regional ischemia and 10 min of reperfusion. The dose-effect relationship and the role of ceruloplasmin conformational integrity in cardioprotection were established by treatment of ischemic hearts with ceruloplasmin at various concentrations (0.25, 0.5, 1, and 2 microM) and at different degrees of conformational integrity (A610/A280 = 0.02, 0.04, and 0.06), 5 min before reperfusion. Deferoxamine (20-500 microM) was used as a positive control. As negative controls we used chemically inactivated ceruloplasmin (1 microM), heat-denatured ceruloplasmin (1 microM), and albumin (1-4 microM). In the control group during the first 5 min of reperfusion, the incidence of total ventricular fibrillation was 100% and of irreversible ventricular fibrillation was 83%. The incidence of reversible and irreversible ventricular fibrillation was significantly decreased in the ceruloplasmin-treated groups in both a dose and molecular integrity dependent manner. Ceruloplasmin had no effect on the incidence of ventricular tachycardia. Deferoxamine reduced the incidence of ventricular fibrillation to the same degree as ceruloplasmin but at concentrations much higher than those of ceruloplasmin. Chemically inactivated ceruloplasmin, heat-denatured ceruloplasmin, and albumin had no protective effects on reperfusion-induced arrhythmias.