The addition of lactated Ringer's solution which contains calcium (RL) to citrated blood products poses a potential risk whenever the level of ionized calcium reaches a concentration capable of catalyzing the coagulation cascade. RL solution is used extensively as both a replacement and a maintenance fluid in the operating room. However, major surgical procedures often require replacement of blood components as well, during the operation. This study examines the in vitro propensity for coagulation when red blood cells (RBC) are diluted with RL. Seven mixtures with different ratios of RBC to RL were prepared from each of 23 units of RBC. These mixtures were analyzed for ionized calcium, total calcium, and pH, and were checked for any indication of coagulation. Nineteen additional RBC units were split into two parts which were mixed with equal volumes of normal saline (NS) or RL. These mixtures were filtered (40 micron) and the weight gain of the paired filters compared. From these studies, the authors identified a threshold value for ionized calcium (0.23 mM/L) below which the probability of clot formation is less than 0.01. This concentration is not reached if the RBC to RL volume ratio is 2:1 or greater. As much as 100 ml of RL can be added to a unit of RBC without exceeding the threshold value. Although not advocating the routine use of RL as a diluent for RBC, the authors conclude that, within the guidelines described, calcium containing salt solutions may be used to dilute blood products.