A bioreactor system for culturing cells, comprising: —at least one bioreactor (3); and —at least one hydrocyclone (5a; 5b; 5c; 5d, 5d′; 5e) comprising: —a cell culture inlet (7) which is connected to a cell culture outlet (9) of the bioreactor (3) via a pump (11), whereby a cell culture from the bioreactor can be transferred to the hydrocyclone (5a: 5b; 5c; 5d, 5d′; 5e) when a cell culture is provided in the bioreactor, —a hydrocyclone separation part (13) into which cell culture can be introduced from the cell culture inlet (7), —a top outlet (15) through which a part of the cell culture with decreased cell concentration can be transferred after separation in the hydrocyclone separation pan (13); and —a bottom outlet (17) through which a pan of the cell culture with increased cell concentration can be transferred after separation in the hydrocyclone separation part (13), wherein said bottom outlet (17) of the hydrocyclone (5a; 5b; 5c; 5d, 5d′; 5e) is provided in a position such that when the bioreactor system is used for culturing cells the bottom outlet (17) is in connection with a headspace (21) of the bioreactor (3) and wherein said bottom outlet (17) is provided such that the bottom outlet (17) has a free space around it such that a cell culture transferred out form the bottom outlet (17) is allowed to freely discharge around the bottom outlet.