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Blood loss during cardiac surgery measured by gravimetric, colorimetric, patientweighing, and red cell volume studies, has been compared. It is concluded that all methods agree closely, but that the red cell volume studies suggest a "concealed" blood loss at the operation site, not measured by other methods. The colorimetric method described here offers a simple and practical means of measuring blood loss during surgery and has been introduced as routine during all major surgical procedures requiring blood replacement. It is emphasized that the blood loss measured in this way is the minimum loss and represents the minimum amount of whole blood that should be replaced by transfusion.