Fit plain end culture tubes and flasks with DeLong style necks or ports. Molded from durable, autoclavable polypropylene (PP). Ridges inside cap provide secure grip and gas exchange. Available in five (5) colors for easy sample identification and organization. Maintain sterility without cotton plugging. Reduce sample evaporation compared to cotton plugging. Provide same evaporation and sterility benefits as stainless steel closures.