It is common practice in many cardiovascular surgical centers to withdraw 10% to 15% of a patient's blood volume before instituting cardiopulmonary bypass, for later transfusion. The autologous blood units are stored in the operating room at an ambient temperature of 20 to 22 C for periods of up to five hours. Currently accepted blood banking standards require immediate refrigeration of donated blood units. This requirement is based on the assumption that during the procedure of obtaining the blood it is likely that bacterial contamination may have occurred and large colonies of bacteria might result at warmer storage temperature. One hundred units of autologous blood were cultured (89 units from jugular lines and 11 units from radial artery lines). The mean storage time at ambient temperature was 2.9 hours (range 1.5 to 4.8 hours). All units except two were negative for growth of microorganisms at 15 days. These two units cultured a diphtheroid, Corynebacterium acnes (a common skin contaminant), on the ninth day of culture. Both units had been drawn from the same patient via an external jugular neck line prepared with 70% alcohol and had been stored at ambient temperature for 4.8 hours. (This patient had had coronary artery bypass and had remained afebrile throughout an uneventful postoperative course, with no signs of sepsis at four months' follow-up.) The results of this study indicate that bacterial growth in autologous blood units stored at ambient temperature is indeed possible. Strict aseptic technic must be observed in placing the lines through which the blood is collected. The potential benefits (eg, platelet viability) versus hazards of storage at ambient temperature are reevaluated.