Method of separating cells in a centrifuge, preferably for producing platelets. A cell suspension is provided in a processing bag and is centrifuged. The cell enriched fraction is transferred to a storage bag via an outlet tube. The outlet tube is arranged with a portion thereof in a position adjacent the periphery of the centrifuge. When the enriched fraction is transferred through the outlet tube placed at the periphery of the centrifuge and during centrifugation, the outlet tube operates as a cell trap and the enriched fraction is further separated. Preferably, the cell suspension is buffy coat and the enriched fraction comprises platelets. The corresponding bag assembly comprises a processing bag intended to enclose the cell suspension. The processing bag is placed in a centrifuge rotor for separating the cells into a fraction enriched on platelets by centrifugation. An outlet tube transfers the enriched fraction to a storage bag and is placed in a position adjacent the periphery of the centrifuge rotor for further separation of the enriched fraction at transferring, during continued centrifugation, the enriched fraction through the outlet tube.