Abstract: Microaggregate blood filter adapted to pre-filter aggregates larger than 150 microns without interfering with the on-going and continuous filtration of aggregates between 40 and 150 microns. Filter comprises a flexible housing having inlet and outlet ports and an interior nest of two generally cylindrical concentric outer and inner filter screens with a space between the screens. Blood to be filtered enters the housing from the top and is pre-filtered for aggregates larger than 150 microns by the inner screen. The inner screen defines a relatively large interior volume which allows filtered aggregates to collect by sedimentation at its lower portions in a manner that permits on-going, continuous, and unobstructed filtration of the blood through its upper portion for subsequent and final filtration through the outer 40 micron filter. A preferred filter has a transparent housing and built in drip chamber and can be primed without inverting the filter.