The aim of the study was to monitor the erythrocyte deformability as one of important factors securing the appropriate tissues perfusion in healthy subjects and diabetic patients with insulin dependent diabetes mellitus. Erythrocyte deformability was determined by the method of filtration and centrifugation, and the erythrocyte filtrability was calculated as a percentage of filtered erythrocytes out of the number of erythrocytes counted before centrifugation. Diluted blood suspensions were filtered by centrifugation through membrane filters with pores of 5 microm in diameter. The speed and duration of centrifugation of 1400 rpm and 5 min respectively were selected as the best ones for filtration due to the lowest value of the coefficient of variance. The values of the arithmetic mean and standard deviation of erythrocyte filtrability were 72.2 +/- 7.9% in normal subjects. In the group of diabetic patients with the long history of insulin therapy the values amounted to 69.1 +/- 4.4%. In diabetic patients, the average value of blood glucose was 11.7 mmol x L(-1), and the glycated haemoglobin concentration reached 9.04%. From the viewpoint of reference values, these facts indicate good compensation of diabetes mellitus. Other haematological values, namely erythrocyte count, haematocrit value, haemoglobin concentration and mean cell volume were within normal reference ranges. No difference between the groups of healthy subjects and diabetic patients was found. (Fig. 4, Ref. 18.)