Long-term engraftment was obtained in recipients. We have followed up monitoring the level of platelet-FVIII activity and the inhibitor titer in immunized recipients after BM transplantation. (A) Sustained platelet-FVIII expression in recipients. Platelets were collected from recipients at various time points. Platelets were lysed with 0.5% CHAPS and the platelet-FVIII:C was determined by chromogenic assay. (B) Inhibitor titer in recipients. Plasma was collected from recipients at various time points, and the inhibitor titer was determined by Bethesda assay. The result shows that preexisting FVIII immunity does not abrogate long-term reconstitution of megakaryocytes expressing FVIII in BM transplants.