monocytes - originate in marrow, spend up to 20 days in the circulation, then travel to the tissues where they become macrophages. Macrophages are the most important phagocyte outside the circulation. Monocytes are about 9% of normal wbc count. They undergo chemotaxis, phagocytosis, and intracellular killing in much the same manner as neutrophils. In addition macrophages act as antigen-presenting cells for T-lymphocytes. They are capable of cellular cytotoxicity and secrete certain cytokines.