The in vitro effect of fresh frozen plasma (FFP) on the whole blood activated coagulation time (ACT) was examined in 18 patients undergoing cardiopulmonary bypass (CPB) during coronary artery bypass graft surgery. The addition of FFP to whole blood in vitro, after systemic heparinization, significantly prolonged the ACT from 451 +/- 21 seconds (mean +/- SE) to 572 +/- 41 seconds (P less than 0.05). There was no significant correlation between the plasma antithrombin III activity and the prolongation in ACT after systemic heparinization, with or without addition of FFP. The addition of FFP to whole blood in three of the six patients who exhibited heparin resistance (ACT less than 400 seconds after administration of 350 unit/kg heparin) did not prolong the ACT to greater than 400 seconds. These observations suggest that infusion of FFP will further prolong the ACT after heparin administration in most patients including some with initial heparin resistance.