A reversible liquid filtration system for cell culture perfusion comprises: a bioreactor vessel (B4), for storing the cell culture (L4); a perfusion pump (P7), comprising a reciprocable element (P71) which is movable in opposing first and second pumping directions (dF, dR); a filter (F4); and first and second bi-directional valves (BV1, BV2), each selectively controllable between open and closed positions. The perfusion pump (P7), the filter (F4), and the first and second bi-directional valves (BV1, BV2), together comprise a fluidic circuit in communication with the bioreactor vessel (B4). The bi-directional valves (BV1, BV2) are controllable to open and close in co-ordination with the reciprocating perfusion pump (P7), in order to enable both a two-way filtering flow around the fluidic circuit and also an alternating filtering flow between the bioreactor vessel (B4) and the perfusion pump (P7).