The method for the holding in flotation of a substance such as a tissue part in a bioreactor (61) is characterized in that the substance (73) is acted upon with a fluid; and in that the flow of the fluid acts counter to gravity in such a manner that the substance (73) is held in flotation. The bioreactor (61) with a container (62) for a substance (73) which is to be acted upon with fluid comprises a first flow chamber (66a) to which a flowing fluid can be supplied, with the first flow chamber (66a) being designed in such a manner that the fluid which flows upwardly therein has a lower speed with increasing height.