The hypothesis tested was that free radicals generated following ischemia and reperfusion in cardiac operations can produce clastogenic factor that results in chromosomal aberration. Fourteen randomized patients undergoing coronary artery bypass grafting were divided into two groups. In Group 1 (7 patients), myocardial protection was achieved using a cardioplegic solution without allopurinol. In Group 2 (7 patients), 100 mg of allopurinol (xanthine oxidase inhibitor) was added to the solution. In both groups, blood samples were taken from the coronary sinus before the aorta was clamped and 20 minutes after myocardial reperfusion was achieved. The blood samples were used to study the patients' chromosomes. The results were given as the percentage of chromosomal aberrations observed in 100 mitoses. There were no significant differences between the preischemic values in both groups and the postischemic values in Group 2. On the other hand, there was a significant difference between the postischemic values in Groups 1 and 2 (p less than 0.01). In conclusion, reperfusion following myocardial ischemia in cardiac operations can produce clastogenic aberrations. This clastogenic activity can be reduced by adding allopurinol to the cardioplegic solution.