Despite recent studies, the organization of coagulation factor VIII (FVIII) on a phospholipid (PL) membrane is not known in detail. Thus, 2-dimensional (2D) crystals of human FVIII lacking the B domain were prepared for electron microscopy onto negatively charged PL monolayers. The 3-dimensional (3D) density map of the PL-bound FVIII protein was calculated at 1.5 nm. Existing atomic data and models for FVIII domains were fitted unambiguously within the 3D density map of the molecule. FVIII domains arrangement followed a compact spiral organization with the A3 domains in close association with the C1 and C2 domains near the PL surface. Viewed toward the membrane the A domains' heterotrimer is oriented side-on with the pseudo-3-fold axis almost parallel to the PL surface and A1 fully covering C1. The C2 domain is partially overlapped by the A2 domain of an adjacent molecule in the 2D crystal, favoring close packing. Viewed parallel to the membrane, C2 is slightly inclined to the PL surface covering an area of 12 nm(2). Four C2 loops are embedded within the lipid monolayer at about 0.7 to 1.0 nm depth. C1 forms almost a right angle with C2, its long axis nearly parallel to the membrane. The proposed structure for membrane-bound FVIII results from modeling of the FVIII domains within a 3D density map obtained from electron crystallography and accords with the main biochemical and structural information known to date. A model is proposed for FVIIIa and factor IXa assembly within the membrane-bound factor X-activating complex. (Blood. 2002;99:1215-1223)