Activated polymorphonuclear leukocytes (PMN) secrete lysosomal enzymes, eicosanoids and toxic oxygen metabolites. In cardiac surgery patients, we measured arterial plasma levels of PMN and L1 (calprotectin), a prominent granulocyte protein, during cardiopulmonary bypass (CPB). The myocardial arterio-venous gradients were evaluated during reperfusion after cold cardioplegic arrest (n = 10). The arterial plasma concentration of L1 increased during CPB from 344 +/- 71 micrograms/l (mean +/- SD) preoperatively to 5221 +/- 1267 micrograms/l at the end of CPB (P less than 0.001). Simultaneously, the number of circulating PMN also increased (from 4.4 +/- 0.4 x 10(9)/l to 9.1 +/- 1.2 x 10(9)/l (P less than 0.05)). There was a positive correlation between the mean number of circulating PMN and the plasma level of L1 at all sampling times during CPB (r = 0.93, P less than 0.05). A coronary sequestration of both L1 (P less than 0.006) and PMN (P less than 0.002) was found after 5 min reperfusion. This was not present at 15 and 30 min reperfusion. The coronary entrapment of L1 and PMN did not covary significantly, and was unrelated to both the time of cardioplegic arrest and the arterial levels of L1 and PMN. In conclusion, the increased plasma concentrations of PMN and L1 during CPB and the coronary sequestration of both PMN and L1 may be factors in the pathogenesis of reperfusion injury of the myocardium.