Abstract: Apparatus for continuously monitoring the loss of blood suffered by a patient in the course of a surgical operation. The apparatus provides a means for sucking the combination of blood and irrigating solution gathered about the incision, and for sequentially measuring the volumes and densities of small samplings of the solution-and-blood mixture being drawn through the conduit leading from the incision area to the suction device. The apparatus relies on the difference between the respective densities of the blood and of the irrigating solution in order to calculate the exact volume of blood contained in each sampling. The blood loss is continuously accumulated and displayed on a visual read-out, so that a corresponding amount of blood can be transfused to the patient in order to maintain his blood pressure throughout the operation.