Bone marrow is the spongy material found in the center of many bones in the body. The bone marrow is the substance that makes the different types of blood cells. A bone marrow aspirate and biopsy is done to see if the blood cells in your child's body are being properly made. Aspiration refers to the removal of fluid by suction. A biopsy is the removal of a small piece of marrow tissue by a special biopsy needle. Most of the time, this procedure is done under general anesthesia either in the clinic or in the operating room.