Monocytes in peripheral blood are relatively well differentiated from lymphs and granulocytes by the combination of forward and orthogonal scatter measurements. At least in mammals, cells of the monocyte/macrophage lineage contain esterases not present in lymphs and granulocytes; Technicon's original Hemalog D flow cytometric differential counter (1972) used a colorogenic esterase substrate for monocyte counting. However, the reaction product was detected by absorption and scattering. Years ago, I tried fluorescein dicaproate as a fluorogenic substrate for esterase, and I seem to remember it working well enough to discriminate monocytes from other peripheral blood leukocytes. I believe the material came from Research Organics; if they don't have it, they may have another fluorogenic substrate for esterase. Given the number of different species in which you need to count macrophages, it is unlikely that you would find suitable antibodies for all of them, and even less likely that you'd find one which detected macs across broad species barriers, so scatter and esterase are probably your best bets. -Howard
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