Abstract: A filter for blood comprising a pair of members together forming a housing with an inlet and an outlet, an aperture diaphragm between said members across the interior of said housing and dividing said housing into a filter chamber and a drip chamber respectively communicating with said inlet and outlet, means forming a hermetic and sterile seal between said members, and a plurality of filter elements serially disposed within said housing so that blood being filtered therethrough must pass in sequence through said inlet, through all of said filter elements, through the aperture of said diaphragm and then through said outlet. Preferably at least four woven nylon screens are employed as the filter elements in a nylon housing, the filter elements in the direction of flow successively having screen sizes of about 200 .mu., 50 .mu., 20 .mu. and 10 .mu..