The sterility of previously assembled cardiopulmonary bypass circuits was investigated for 17 extracorporeal circuits. The closed circuits were assembled using aseptic technique and remained in the operating room until the time of use. The mean time from point of setup to point of priming for the 17 randomly chosen circuits was 21.47 hours, with a range of 13.50 to 60.50 hours. Circuits were primed with three liters of sterile Plasma-Lyte A Injection, circulated for 5 minutes and tested for microbial contamination by withdrawing one liter of the priming solution through an Addi-Chek Quality Control System. The Addi-Chek canister, which contains a 0.45 μm cellulose membrane filter, was then filled with tryptic soy broth and incubated for 14 days. All were found to be free of microbial contamination as indicated by no growth in culture. The results of this investigation demonstrate that the sterility of the extracorporeal circuit, pre-assembled in advance of actual priming, can be maintained over an extended interval when standard aseptic technique is used. This allows the utilization of a pre-assembled circuit for emergency cardiopulmonary bypass support.