Quantitative evaluation of FVIII:C levels in recipients' platelets. Platelets were isolated from recipients after 4-week BM constitution and lysed in 0.5% CHAPS. The levels of platelet-FVIII activity were determined by chromogenic assay. Bars represent mean plus or minus SD. Statistical analysis by 2-tailed Student t test revealed a P value less than .01 between 1-month and 2-month FVIII levels when 660 cGy TBI was used. A value of P less than .05 was considered statistically significant. Data shown are summarized from 10 BM transplantation trials. This result demonstrates that 2bF8 genetically modified cells of the megakaryocytic lineage are successfully engrafted into hemophilic mice with preexisting immunity.