Whole blood gravity sedimentation technique can be modified for studying leukocyte sedimentation properties. Previously, we demonstrated that the displacement rate of leukocytes was associated with activation of leukocytes during traditional gravity sedimentation of the whole blood. The plasma flow as well as the difference between the specific gravity of leukocytes and plasma propel the leukocytes upward in the sedimentation tube while the erythrocyte aggregates are descending. The leukocyte ascension rate can be described as the increment of leukocyte concentration in the upper half section of the blood column after one-hour sedimentation. The aim of the present study was to characterize the ascending and non-ascending leukocytes using a flow cytometric technique. Venous blood samples were taken from 8 healthy controls and 8 septic patients after major thoracic or abdominal surgical procedures. The upper and lower halves sections of venous blood column were separately removed from the sedimentation tube after one hour gravity sedimentation. Using flow cytometry, the leukocyte subsets were identified by their CD45 density and side scatter parameters followed by characterization of their cellular size and cytoplasmic granularity. The size indices of septic patients' ascending polymorphonuclear leukocytes (PMNs) were significantly lower than that of the non-ascending ones (253 +/- 22 versus 387 +/- 12 (SEM), p < 0.002) or the ascending PMN fraction taken from healthy individuals (382 +/- 28, p < 0.005). Septic patients' ascending PMNs presented significantly lower cytoplasmic granularity indices compared to non-ascending (447 +/- 23 versus 538 +/- 18, p < 0.05) or healthy ascending PMNs (539 +/- 20, p < 0.05). The cellular size and cytoplasmic granularity indices of heavy and light monocytes as well as lymphocytes were similar in both groups. It can be assumed that venous blood samples of septic patients contain significantly smaller PMNs with less cytoplasmic granularity than healthy control cells.