Thumb Caps fit 12mm plastic test tubes. Protects samples from accidental spillage, evaporation and contamination. Flange Caps fit 12mm and 13mm test tubes, round cuvettes, and centrifuge tubes. Flange Caps have two flexible flanges to assure a leakproof seal. Hollow Bottom Plug Caps fit 16mm test tubes. Hollow Top Plug Caps fit 12mm, 13mm, 16mm, and 17mm test tubes. All caps are made of LDPE. Caps listed here are white in color. Other colors are available upon request.