Stored human blood of varying age was passed through standard commercial blood transfusion filters (pore size, 170 mu) and Swank Dacron wool blood transfusion filters (pore size, 20mu). Passage through the Dacron filter resulted in a marked decrease in SFP and an increase in filter weight indicating removal of microaggregates which have been implicated as a cause of pulmonary insufficiency. The commercial filter tested did not appear to be effective in removing these harmful aggregates. On the basis of this research it is concluded that the Swank Dacron wool blood transfusion filter will prevent pulmonary microemboli during transfusion. Because aggregate removal appeared to be quantitative, it is recommended that no more than four units of blood be passed through each Dacron wool filter.