A method of cultivating cells in a bag is disclosed, which comprises the steps of:
a) providing a sterile flexible bag comprising a filter material which is attached to a wall of the culture bag, wherein the filter material delimits a chamber inside the bag and wherein the chamber is fluidically connected to a first port in a wall of the bag,
b) providing agitation means for the bag,
c) introducing cell culture medium, microcarriers and cells to the bag,
d) cultivating cells in the bag with agitation provided by the agitation means, allowing formation of a suspension of microcarriers with immobilized cells and supplying at least one gas via a second port (not shown) in a wall of the bag,
e) removing cell debris and/or free cells together with liquid from said suspension of microcarriers through the filter material and the first port.