Eleven patients were given varying doses of cardioplegic solution contaminated with Enterobacter cloacae. Five patients died. Early bleeding, necessitating reoperation, occurred in eight patients and a total of 126 units of blood and 203 units of platelets were given (range 2 to 19 and 15 to 47 units, respectively). Mycotic aneurysms developed in four patients, rupturing between the ninth and eleventh postoperative day. Only one of these patients survived. Other complications included adult respiratory distress syndrome (three patients), renal failure (four patients), sternal infections (six patients), and organic brain syndrome (five patients). Although some factors of gram-negative septicemia were identified in retrospect, others were masked by the clinical setting in which it occurred. We recommend that each dose of cardioplegic solution be prepared on an individual basis and used immediately. We also recommend that 'sternal blood' be cultured on all patients. The subtle features of 'gram-negative septicemia' necessitate urgent investigation and treatment. The combination of low white cell count, high cardiac output, and low peripheral vascular resistance should be assumed to indicate septicemia until proved otherwise. A full coagulation screen including platelet function and fibrin degradation products should be performed in any and all patients with these findings. Mycotic aneurysms mandate urgent reoperation with interposition of a saphenous vein segment if these patients are to survive.