Flow rates of compatible mixed packed red cells passing through a 170 micrometers standard blood filter or one of three microfilters with different pore sizes were measured. In spite of the largest pore size of 170 micrometers, the standardfilter shows the slowest flow. The cascade filter "MF 10" with sieves of 200, 50, 20, ad 10 micrometers pore sizes has excellent flow rates due to its large surface area. The "microtrans" with cascades of 150, 50, 10 micrometers pore sizes, good filter properties, low filling volume, and reasonable flow rates seems to be an acceptable microfilter for routine clinical use. If the tube of the infusion system is cut off, then the blood flow increases its speed by about fivefold in spite of different filters.
