For the preparation of 1 Liter liquid for LB agar plates weigh 10 g tryptone, 5 g yeast extract, 5 g NaCl and 15 g agar. Add 1 mL 1 N NaOH to the dissolved powder. The medium will be autoclaved for 25 minutes. At a temperature of ~50°C, it is still liquid and additives (e. g. antibiotics) can be added. If 32 to 40 mL LB agar per plate are used, one liter will result in approx. 25 to 30 plates. Freshly poured plates cool down with a slightly opened lid and turn them upside down when stored at +4°C.