Intraoperative autotransfusions for selected patients can ease demands on blood banks. In 33 cases (trauma, elective vascular, and cardiac procedures), we saved 80.7 L of blood with a cell washer blood recovery system (Cell Saver®). In eight trauma cases, including three with intestinal perforation, cell washing salvaged 31.5 L of shed blood as a lifesaving emergency procedure. This stimulated interest in defining the limits of the instrumentation in cleansing blood of bacteria. In 18 in vitro experiments, discarded banked blood was inoculated withEscherichia colito simulate light, moderate, or heavy bacterial contamination. Volumes of blood (500 ml) containing a total of 6.3 × 105, 4.8 × 107, or 3.2 × 1010bacteria were processed. The original blood, filtered blood, plasma supernate, effluent wash solution, and final washed packed red cells were cultured quantitatively in each experiment. The mean total number ofE. coliretained in the final washed packed red cells was 1.4 × 105, 6.3 × 106, or 1.6 × 102bacteria, or 23%, 13%, and 5% of each original inoculum. Additional washing with 10 L of saline did not remove significantly more bacteria (p> 0.20). Although emergency autotransfusion of blood contaminated with intestinal contents was lifesaving, we recommend caution since these results show that cell washing does not remove all bacteria. Further laboratory and clinical studies are needed to determine the levels of bacterial contamination of autotransfused blood that can be tolerated, and to determine adjunctive means of rendering contaminated or potentially contaminated blood safe for autotransfusion.