Abstract: Centrifuge tube cap has a plug which fits into the open end of the tube. A reduced diameter stud on the plug has an axial bore and extends outside of the tube thereby forming a shoulder at the lip of the tube. The peripheral portion of the shoulder is beveled. An annular cap is placed over the open tube end and stud portion of the plug and secured thereto by a nut which, when tightened, wedges the tube wall between the cap and the shoulder portion of the plug. The cap has a downwardly extending skirt whose inner wall is flared outwardly. This wedging action compresses the wall of the tube between the plug and the cap thereby forming a seal. A post fill screw is threaded into the central bore formed in the plug.