Two histochemically distinct populations of fat cells are present in the bone marrow with a topographic distribution paralleling that of hemopoiesis. Fat cells of red marrow stain positively with performic acid-Schiff (PFAS) whereas those of yellow marrow do not stain with this technique. When hemopoietic tissue expands in response to experimentally induced hemolysis, PFAS-positive cells of red marrow disappear whereas the PFAS-negative cells of yellow marrow remain stable. This differential staining indicates a difference in lipid composition of fat cells in red and yellow marrow and may relate to their relative stability during the expansion of hemopoiesis.