Abstract: Blood loss by a patient during an operation is monitored by collecting all blood and bloodied materials generated during the operation and subjecting them to ultrasonic vibration in a buffered eluent containing a volume control substance by which the actual volume of the blood-added eluent can be determined. Hemoglobin concentration in the eluate is measured periodically or continually, and multiplied by the eluate volume, to give the actual hemoglobin lost. This value is adjusted by the hemoglobin concentration of the patient just prior to the operation, to give the actual blood volume lost. The blood lost during the operation can also be monitored continuously, and real time values provided.