A device for controlling the supply of red blood cells to a patient in a transfusion line, comprising a blood collection tank (1) connected by means of a duct (2) to a tank (3) for infusion to the patient or directly to the patient, the device being provided with means (9, 10) that are adapted to detect the concentration of red blood cells in the blood contained in the collection tank (1) by way of a detection performed on the tank or on the duct (2) that connects the collection tank (1) to the tank (3) for infusion to the patient or directly to the patient, these detection means being adapted to actuate fastening means (14, 15, 16, 17) that are adapted to selectively close and open the duct (2) in output from the collection tank (1) so as to block or respectively allow the passage of the liquid flow through it, the detection means adapted to detect the concentration of red blood cells in the blood contained in the collection tank (1) being adapted to measure the degree of absorption of a luminous radiation that passes through the blood.
