L-15 Medium (Leibovitz) was formulated for use in carbon dioxide (CO2) without the requirement of sodium bicarbonate/CO2 buffering systems. This is achieved by the use of free basic amino acids, phosphate buffers and higer levels of galactose and sodium pyruvate. This allows for this medium to be used under conditions of free gaseous exchange with the atmosphere. The medium has been shown to work for human tumor cells and embryonic cells and for established cell lines such as HeLa and Hep-2. Diagnostic virology uses this medium frequently due to the use of closed systems. L-15 medium obviated the need of frequent medium changes.