The MTC Bio C2603 50mL Polypropylene Centrifuge Tubes, Flat Screw Cap, 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.