The MTC Bio C2605 50mL Polypropylene Centrifuge Tubes, Flat Screw Cap, Free Standing, 25 per Sterile Bag, 29 x 115mm, Pack of 500, are made of premium polypropylene for clarity and centrifugal strength. The tubes and caps may be frozen at temperatures as low as -86˚C. The maximum centrifugal force is 15,000xg for the 50 ml tubes. The caps have a flat top surface for easy marking and labeling. In addition, the tubes each feature a 'skirted' bottom, which allows an individual tube to stand upright without a separate test tube rack.