Tubes: made of strong, highly transparent medical grade polypropylene Flat-top caps: made of medical grade polyethylene Large white writing area for easy marking & flat-top caps can be written on Double threaded design reduces cross-threading; caps are easily opened and closed Gamma radiation sterilized, non-pyrogenic RNase & DNase free Specifications 50ml can withstand centrifugation of 9,400RCF Sealing test: complies with The International Air Transport Association (IATA) Safety Standards