Marrow is a loosely bound tissue in which hemopoiesis has frequently been considered to be randomly distributed. The case is presented, however, for an organized and structured marrow in which close relationships exist between hemopoietic tissue and a regulatory microenvironment. Distributions of myeloid cells in the mouse femur are described, and a dynamic picture of their movement, with differentiation and maturation from the endosteal surface of the bone to their release via the central venous sinus, is painted. It is also shown that this structure is established within three weeks of birth. By contrast, mature lymphoid cells (but not their progenitors) are uniformly distributed. Regulatory stromal elements in the marrow are also structured and their localization is found to correspond closely to the properties of the progenitor populations. Such structure has potential practical importance, particularly in the field of medical, industrial or accidental radiation exposure where bone may introduce non-uniform dose distributions in the marrow.