The MTC Bio C2601 15mL Polypropylene Centrifuge Tubes, Flat Screw Cap, 50 per Sterile Bag, 17 x 118mm, 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 12,000xg for the 15ml tubes. The caps have a flat top surface for easy marking and labeling.