Objective. To establish a flow cytometric internal standard method for counting platelet-derived microparti-cles （PMPs） and to study its clinical significance.Methods. PMPs suspension （platelet poor plasma

PPP） was extracted by gradual centrifugation. According to the size of PMPs

3 μm and 0. 8μm latex beads were used as internal standards for the quantitation. PMPs were counted by adjusting flow cytometric discrimination and voltage of forward scatter and side scatter.Results. In 30 healthy donors

the average concentration of resting PMPs was （1. 2×105±5.7×104 ） /ml and that of activated PMPs was （1. 6×106±9. 1×105） /ml. Compared with healthy donors

PMPs mean value was significantly higher （ P < 0. 001） in 18 patients with coronary artery disease

12 with acute cerebral infraction and 23 with chronic renal failure [the average PMPs concentration

（ 6. 1×105±2. 5×105 ）/ml

（ 6. 8×105±3. 4×105） /ml and （5. 9×105±3. 1×105） /ml respectively]. However

no significant difference in PMPs concentration was observed in 25 patients with acute leukemia and severe thrombocytopenia during the aplastic phase after chemotherapy [ （1. 3×105±6. 1× 104） /ml

（ P > 0. 05） ] .Conclusions. PMPs is a useful indicator in monitoring platelet activation

and plays an important role in thrombotic disease. By flow cytometric internal standard method

PMPs can be counted rapidly and accurately

which may be very helpful in interlaboratory comparative studies.