Packed erythrocytes (PC) are used to treat intraoperative blood loss and frequently re- constituted with normal saline (NS) to overcome the slow infusion rate due to high viscosity and inadequate volume replacement. It has been recommended that calcium containing solutions such as Hartmann's solution (HS) should not be used as a diluent of PC, citing possible clot formation whenever the level of ionized calcium reaches a concentration capable of catalyzing the coagulation cascade. We studied in vitro coagulation propensity, if any, by analyzing the mixtures of PC with different amounts of HS or NS in ratios between 10: 0 to 1: 9 (PC: crystalloids). These mixtures were analyzed for ionized calcium concentration, pH, Hct and checked for the gross indication of clot formation at room temperature. No clot formation was found in NS-reconstituted samples, however, 55 of 100 HS-reconstituted samples were present with clot formation. In these samples, the initial clot formations were found at ratios of 6: 4 (1case at 45 min), 5: 5 (1 case at 45 min, 1 case at 60 min, 3 cases at 90 min), 4: 6 (1 case at 45 min, 3 cases at 60 min) and the mean ionized calcium concentration of initial clotting was 0.304+/-0.089 mM/L In conclusion, as much as 100ml of Hartmann's solution can be safely usedas a diluent of one unit of PC in patients requiring rapid blood transfusion.