Abstract: A blood collection device includes a tube for receiving a sample of whole blood for centrifugal separation into the lighter phase, plasma or serum, and the heavier cellular phase. The tube has a needle-pierceable stopper at the opposite end for maintaining a negative pressure in the tube. A blood phase partitioning device is disposed within the tube and includes upper and lower members with the upper member having a greater specific gravity greater than that of the lower member but together having a specific gravity between that of the lighter phase and that of the heavier phase. A sealant is disposed between the members and in contact with each member and has a specific gravity equal to the average of the two members.