Abstract: Blood is withdrawn from a patient for ultimate centrifuging into plasma and erythrocytes by mounting a supple substantially non-extensible under conditions of use thermosealable walled container in a carrier, the container having a top provided with an orifice closed by a rubber washer. A needle is mounted in a hollow cap sized to receive the carrier. The needle is inserted in the vein by pressing the carrier into the cap which also causes the needle to perforate the washer. A vacuum is drawn on the carrier drawing blood into the container, the container having a medial restriction dividing the container into two equal chambers. After filling, the container is withdrawn from the cap before removal of the needle from the vein and the container is mounted in a second carrier which is closed by the cap. The blood is then centrifuged, the plasma collects in the chamber above the restriction and the erythrocytes collects in the chamber below the restriction.