Borosilicate glass culture tubes made from tubing with uniform wall thickness for maximum heat transfer and chemical resistance. Thread finish complies with Glass Packaging Institute (GPI) requirements. Cap included. The 20x25mm size is suitable for the determination of insulin according to the method of Alving, Rubin and Millor, Journal of Biological Chemistry, 127, 3 (March 1939). Designed from ASTM E982, Type VI, Class A requirements. Replacement cap ref. 45066C (PTFE coated liner).