Cancellation of on-pump coronary artery bypass grafting after the circuit is primed may result in the discarding of unused circuits. In some off-pump cases, a surgeon may request that the circuit be primed, but complete the surgical procedure without utilizing the circuit. The major concerns about the unused circuit are its sterility and the performance of the oxygenator after it has been primed for a long period of time. The goal of this study is to determine whether prepriming of the circuit with and without albumin has an effect on the gas transfer efficiency of oxygenators during simulated cardiopulmonary bypass. Monolyth integrated membrane lungs (Sorin Biomedical, Arvada, CO) were used to deoxygenate and oxygenate the bovine blood. Oxygenators were preprimed for 72 (N = 6) and 24 (N = 6) hours before testing. In control group (N = 6), oxygenators were tested immediately (0 h) after they were primed. Three different priming solutions were used: physiological saline solution (Group A); 1.25% of human albumin (Group B); and 5% human albumin (Group C). The blood was modified to the American Association of Medical Instrumentation Standards before testing. The blood flow through the oxygenators was set at 2 Lpm and 4 Lpm, with gas (FiO2 at 1.0) to blood flow ratio at 1:1. Cultures were also obtained from preprimed oxygenators to test circuit sterility. Oxygen transfer in oxygenators primed for 0 h at blood flow of 4 Lpm were 203 mL/min +/- 9.7 (Group A), 263.1 mL/min +/- 52.9 (Group B), and 270.5 mL/min +/- 13.1(Group C, p < .01 vs. Group A). In oxygenators preprimed for 72 h, the CO2 transfers were 135.0 mL/min +/- 21.8 (Group A), 104.9 mL/min +/- 2.4 (Group B), and 148.9 +/- 26.6 (Group C, p < .006 vs. Group B). In addition, the pressure drops were 56.5 mmHg +/- 5.5 (Group A), 82.6 mmHg +/- 13.4 (Group B), and 67.6 mmHg +/- 15.3 (Group C, p < .05 vs. Group B). In group A, O2 transfer were 203.5 mL/min +/- 9.7 (0 h), 272.4 mL/min +/- 66.6 (24 h), and 260.8 mL/min +/- 31.1 (72 h, p < .01 vs. 0 h). In group B, O2 transfer were 263.1 mL/min +/- 52.0 (0 h), 302.7 mL/min +/- 77.4 (24 h), and 235.2 mL/min +/- 16.5 (72 hr, p < .02 vs. 24 hr). Cultures obtained from 12 preprimed oxygenators presented no organism growth for up to 5 days. In conclusion, oxygen transfer increases in oxygenators preprimed with albumin immediately after they were primed. However, gas transfer decreased after they were primed with albumin for 72 h. Oxygenators preprimed for 24 h and 72 h with 0.9% saline had better O2 transfer than those primed for 0 h.