Saponin susceptibility of the platelet was studied in 10 healthy men (ages 24 to 39) and 31 male hemodialysis (HD) patients (ages 24 to 49) in order to assess platelet abnormalities in HD patients. Platelet-rich plasma (PRP) was added to 50 ml of saline containing 5 mM phosphate buffer (pH 7.0) and 0.3% saponin. Platelet mean particle volume (MPV) was observed continuously for 200 seconds by a continuous mean particle analyzer model CC-108 (Sysmex, Kobe, Japan). Initial MPV (Vo, fl) was 7.12 +/- 0.89 and 7.68 +/- 0.56 respectively for the control and HD patients (p less than 0.05), and maximum MPV (V1) was 9.80 +/- 1.06 and 9.78 +/- 0.83, respectively (n.s.). The time of start of swelling (seconds) was 51.2 +/- 14.6 and 63.4 +/- 15.6 (p less than 0.05) and the time of V1 was 110.4 +/- 15.3 and 112.3 +/- 16.1 (n.s.), respectively. The expansion rates (V1/Vo) were 1.38 +/- 0.05 and 1.27 +/- 0.06, respectively (p less than 0.01). The low expansion rate means that the platelet is less responsive to saponin in HD patients, and this may be related to the cholesterol concentration in the platelet membrane and membrane flexibility. This new method offers a useful way of obtaining information about the platelet.