Activation of coagulation factor X (fX) by activated factors IX (fIXa) and VIII (fVIIIa) requires the assembly of the enzyme-cofactor-substrate fIXafVIIIa-fX complex on negatively charged phospholipid membranes. Using flow cytometry, we explored formation of the intermediate membranebound binary complexes of fIXa, fVIIIa, and fX. Studies of the coordinate binding of coagulation factors to 0.8-lm phospholipid vesicles (25 ⁄ 75 phosphatidylserine ⁄ phosphatidylcholine) showed that fVIII (fVIIIa), fIXa, and fX bind to 32 700 ± 5000 (33 200 ± 14 100), 20 000 ± 4500, and 30 500 ± 1300 binding sites per vesicle with apparent K d values of 76 ± 23 (71 ± 5), 1510 ± 430, and 223 ± 79 nm, respectively. FVIII at 10 nm induced the appearance of additional high-affinity sites for fIXa (1810 ± 370, 20 ± 5 nm) and fX (12 630 ± 690, 14 ± 4 nm), whereas fX at 100 nm induced high-affinity sites for fIXa (541 ± 67, 23 ± 5 nm). The effects of fVIII and fVIIIa on the binding of fIXa or fX were similar. The apparent Michaelis constant of the fX activation by fIXa was a linear function of the fVIIIa concentration with a slope of 1.00 ± 0.12 and an intrinsic K m value of 8.0 ± 1.5 nm, in agreement with the hypothesis that the reaction rate is limited by the fVIIIa-fX complex formation. In addition, direct correlation was observed between the fX activation rate and formation of the fVIIIa-fX complex. Titration of fX, fVIIIa, phospholipid concentration and phosphatidylserine content suggested that at high fVIIIa concentration the reaction rate is regulated by the concentration of free fX rather than of membrane-bound fX. The obtained results reveal formation of high-affinity fVIIIa-fX complexes on phospholipid membranes and suggest their role in regulating fX activation by anchoring and delivering fX to the enzymatic complex.Abbreviations BSA, bovine serum albumin; DiIC16(3), 1,1¢-dihexadecyl-3,3,3¢,3¢-tetramethylindocarbocyanine perchlorate; fVIII(a),