The present invention provides tangential flow filtration devices and methods for enriching a heterogeneous mixture of blood constituents for leukocytes by removal of non-leukocyte blood constituents. In one particular embodiment the device can provide a composition enriched in monocytes. One embodiment includes a remover unit (1) having a crossflow chamber (3) separated by a microporous filter (5) from a filtrate chamber (4), the remover unit (1) also having a tangential flow inlet (6), a fluid outlet (7) for a fluid enriched in leukocytes and a filtrate outlet (8).