By Reid's functional test of erythrocyte deformability, the filtration time of 1 ml 5% erythrocyte suspension through a 5 micron nucleopore membrane was measured from 18 normal people and 19 patients with thalassemia. In the control group, the filtration time at 37.0 degrees C and pH 7.4 was 3.26 +/- 0.38 sec. Erythrocyte deformability was constant while blood temperature stayed between 35.0 degrees C and 42.0 degrees C. However the erythrocyte filtration time prolonged as the blood became alkaline or incubated for more than 2 hours. The filtration time of blood was 4.56 +/- 1.02 sec for thalassemia patients, significantly different (p less than 0.01) as comparing to the normal control group. Poor deformability in diseased blood could be explained by membrane defect. Erythrocyte deformability is a good rheological indicator if the physiological conditions of blood is controlled well.