Pirfenidone regulates fibrocyte pool size in a therapeutic setting. Lung digests stained for CD45 and collagen-I (Col-I) were examined with fluorescence-activated cell sorter analysis. (A) CD45 and Col-I double-positive fibrocytes in lung digests were examined using the logical gates depicted on day 21 of bleomycin (BLM) treatment. (B) CD45 and Col-I double-positive fibrocytes in lung digests from BLM-treated mice administered pirfenidone from day 10 to day 20 were examined on day 21. Pirfenidone administration decreased CD45 and Col-I double-positive fibrocytes from 29.9% to 22.1%. Representative runs are shown for each group. (C) Pirfenidone also regulated the absolute cell number of fibrocytes in the lungs of BLM-treated mice on day 21 in a therapeutic setting. Fibrocytes were increased from 1.3 × 104 cells/mL (saline treated) to 3.2 × 105 cells/mL by BLM treatment and attenuated to 1.1 × 105 cells/mL by pirfenidone administration. Data represent the means ± standard error from 2 independent experiments with 6 mice for each group. PFD, pirfenidone.