Marrow immunostain for transgenic hB-FVIII. Femoral marrow from line no. 38 hB^-FVIII/FVIII^null animals (A,C,D) and from FVIII^null animals (B) was immunostained for hFVIII (A-B) or with a sheep serum control (C). Hematoxylin and eosin stain was done on the marrow from the hB^-FVIII/FVIII^null mice to demonstrate the clear morphology of the megakaryocytes present (D). Only megakaryocytes in the line no. 38 marrow were positive for hFVIIIs. Arrows point to representative mature megakaryocytes in each figure. Original magnification, × 100.