A biological fluid filter assembly comprising: a) a housing having first and second flexible housing walls (14, 16) wherein one of said housing walls comprises an inlet housing wall associated with an inlet port and said other of said housing walls comprises an outlet housing wall associated with an outlet port; b) a filter medium (30, 30', 30") disposed between said flexible housing walls (14, 16), said filter medium (30, 30', 30") comprising at least first, second and third filter elements wherein (i) said first filter element (32, 32', 32") comprises a material suitable for removing a component or particulate of a biological fluid and is disposed between said inlet housing wall and said second filter element; (ii) said second filter element (34, 34', 34") comprises a material suitable for removing another component of said biological fluid and is disposed between said first filter element (32, 32', 32") and said third filter element; and (iii) said third filter element (36, 36', 36") comprises a sheet of an organic or inorganic material and is disposed between said outlet housing wall and said second filter element (34, 34', 34"), characterised in that said sheet comprises a central, porous, mesh portion and an outer peripheral, non-porous, frame portion integral with said mesh portion; said third filter element (36, 36', 36") having an air permeability per 1 cm of thickness of greater than 40 cm 2 /s at a pressure difference of 125 Pa. a) a housing having first and second flexible housing walls (14, 16) wherein one of said housing walls comprises an inlet housing wall associated with an inlet port and said other of said housing walls comprises an outlet housing wall associated with an outlet port; b) a filter medium (30, 30', 30") disposed between said flexible housing walls (14, 16), said filter medium (30, 30', 30") comprising at least first, second and third filter elements wherein (i) said first filter element (32, 32', 32") comprises a material suitable for removing a component or particulate of a biological fluid and is disposed between said inlet housing wall and said second filter element; (ii) said second filter element (34, 34', 34") comprises a material suitable for removing another component of said biological fluid and is disposed between said first filter element (32, 32', 32") and said third filter element; and (iii) said third filter element (36, 36', 36") comprises a sheet of an organic or inorganic material and is disposed between said outlet housing wall and said second filter element (34, 34', 34"), characterised in that said sheet comprises a central, porous, mesh portion and an outer peripheral, non-porous, frame portion integral with said mesh portion; said third filter element (36, 36', 36") having an air permeability per 1 cm of thickness of greater than 40 cm 2 /s at a pressure difference of 125 Pa. (i) said first filter element (32, 32', 32") comprises a material suitable for removing a component or particulate of a biological fluid and is disposed between said inlet housing wall and said second filter element; (ii) said second filter element (34, 34', 34") comprises a material suitable for removing another component of said biological fluid and is disposed between said first filter element (32, 32', 32") and said third filter element; and (iii) said third filter element (36, 36', 36") comprises a sheet of an organic or inorganic material and is disposed between said outlet housing wall and said second filter element (34, 34', 34"),