hCD68-rtTA is activated by monocytes and neutrophils in circulating blood. Flow cytometry was used to assess GFP reporter activation in hCD68rtTA-GFP mice fed doxycycline for 0, 3, 6, 12, or 21 days. A–D: GFP reporter expression by monocytes, neutrophils, eosinophils, and lymphocytes. E–H: representative histograms of GFP expression on day 12 for circulating monocytes, neutrophils, eosinophils, and lymphocytes; day 0 shown in grey. I–L: endogenous murine CD68 expression by circulating monocytes, neutrophils, eosinophils, and lymphocytes; isotype shown in grey. All data are represented as means ± SE; n = 5–6 mice per group.