So that the culture will contain only the desired cells, the medium must be sterile; that is, it must initially contain no living organisms. The most common method of sterilizing culture media is steam sterilization or autoclaving, using steam under pressure. During this process, material to be sterilized is placed in the autoclave and heated to 121°C at 15 psi for 15 minutes.

Containers of culture media such as test tubes or Petri plates should not be opened until you are ready to work with them, and even then, they should not be left open.