Abstract: Apparatus suitable for growth of microorganisms having a tray member with an open top and a closure member capable of being removably placed over the open top of the tray member. The tray member defines a main chamber therewithin and has wall means defining a separate open-topped chamber located within the main chamber, said wall means having an upper edge surface which is substantially coplanar with the open top of the tray member. At least one slot in the wall means provides communication between this separate chamber and the main chamber when the closure member is in place on the tray. When the apparatus is used, a suitable microbiological growth medium is placed in the main chamber and a gas-generating composition is placed in the separate chamber before the closure member is placed on the tray. Thereafter when the closure member is in place, gas from said composition can flow from the separate chamber into the main chamber to provide the desired atmosphere in said main chamber.