An evacuated blood sample collection tube is provided with a stopper carrying a one-way valve for preventing blood drawn into the tube from flowing back to the patient, and for preventing blood trapped in the valve after a sample has been taken, from flowing to the sample during centrifugal blood phase separation. The valve includes an elastomeric valve member and a ring member which moves in response to centrifugal forces to a position in which it applies pressure on the valve member to maintain the valve closed during centrifugation.