Filterability of the blood with various red cell and leukocyte concentrations was examined in 17 patients with coronary heart disease. The blood was filtered through a filter with pore diameter of 5 microns under a pressure of 100 cm water column. Blood filterability drastically deteriorated if leukocyte concentrations in the blood increased and depended on the leukocyte filtration characteristics. The number of red cells passed through the filter was in direct proportion with the blood red cell count.