In order to study the efficiency of third generation blood filters we have devised a new method using 3 microns pore size polycarbonate filter membranes and a filtration chamber to trap leucocytes passing through the blood filter. The method has enabled us to make two important observations: (1) The filters function very efficiently at the start but the efficiency diminishes progressively with time. (2) When the blood flow through the filter is retarded, due to defective priming and/or filter malfunction, the filters fail even at the outset of the transfusion.