A gas-introduction system useful, for example, in biotechnology for supplying cells or microorganisms with oxygen. The gas-introduction system has a bubble column and a distributor. In the vessel there is situated a liquid medium which is to be supplied with gas. A vector is used as transport medium for the gas. The gas is introduced into the bubble column and here taken up by the vector. The vector is applied to the liquid surface via the distributor in droplet form, falls downward in the medium and releases to the medium some of the gas that has been taken up. At the bottom of the vessel is situated a collecting device in which the vector droplets coalesce and pass back into the bubble column. Disclosed are a bioreactor comprising the gas-introduction system and a method for introducing gas into a liquid medium, preferably an aqueous suspension containing cells or microorganisms.