The ultrastructure of bone marrow of rats, mice and guinea pigs was studied after fixation by vascular perfusion. Serial sections were examined to determine the structure and site of pores in lining cells. It was concluded that pores are transient structures occurring only in conjunction with migrating blood cells. These pores occur within lining cells and most frequently near junctions of contiguous lining cells. Microfilaments in lining and adventitial cells are described and their possible roles discussed.