a form of nongranular white blood cells (agranulocytes) that form in the bone marrow. They usually have a kidney-shaped nucleus and a weakly basophilic cytoplasm. In human blood, monocytes normally account for 4–8 percent of the leucocytes. Their diameter is 12–20 microns. Monocytes are capable of phagocytosis and, upon separating from the blood into the tissues, become macrophages.