Corning 15mL clear polypropylene (PP) centrifuge tubes come with the advanced polyethylene CentriStar caps and are packed 50 per rack. The CentriStar Cap has a easy-on/easy-off flat top and offers advanced ergonomics with its wider knurls and roll-over edge design for easier gripping. This leak-proof design comes with a revolutionary plug feature that virtually eliminates all seepage when used under recommended conditions.