Abstract: A method for cultivating cells utilizing wave motion comprises the steps of: providing a container; introducing a gas containing oxygen, a liquid medium and a cell culture into the container; moving the container such that the container swivels with respect to a substantially horizontal pivot axis to thereby induce a wave motion to the liquid medium in the container, which wave motion contributes to the necessary oxygen transfer and mixing required for cell growth. During said swivelling of the container said pivot axis follows a cyclical closed-loop path. Also provided is an apparatus for performing said method, and an assembly for said method. Further part of the invention are disposable bags for cell or bacterial culture of which the volume can be changed in situ by opening or closing compartments of the bag and/or bags with essentially straight corners.