Abstract: In a serum separator, a sampling tube fits inside of a test tube. The sampling tube is provided with an elastomeric piston head at the lower end thereof having an annular lip to engage and form a seal with the inner sidewalls of the test tube. The upper side of the lip is provided with irregularities to break the seal between the lip and the test tube wall upon removal of the sampling tube from the test tube. A porous plastic filter body is mounted in a cavity in the piston head. The piston head defines a one-way valve in the form of a ball chamber containing a ball check. When the sampling tube is pushed into the test tube with a blood sample in the test tube, serum will pass through the filter body and valve into the interior of the sampling tube. The valve is constructed so that the serum cannot squirt toward the top of the sampling tube. When the sampling tube has been inserted, the valve will close.