Add enough active Geneticin to 50 ml of water to make a 50 mg ml−1 stock solution. Each lot of Geneticin will have a different active concentration. For example, if a 5 g bottle of Geneticin has an active concentration of 750 μg mg−1 of powder, it contains an active weight of 3.75 g Geneticin. Therefore dissolve the 5 g of powder in 75 ml of water to obtain an active concentration of 50 mg ml−1. Mix to dissolve, pass through a 0.2 mm filter to sterilize, and dispense aliquots into sterile tubes. Store short term at 4 °C or long term at −20 °C