Objective The unbalanced phenotype of pe-ripheral blood monocyte is closely related to the pathological progres-sion of pulmonary fibrosis .The present study was designed to address the dynamic changes of circulating monocyte subsets in the experimen-tal mouse model of pulmonary fibrosis , and explore the relationship of circulating monocyte subsets with pulmonary inflammation and fibro-sis. Methods A total of 100male C 57BL/6J mice were random-ized as control group and a bleomycin A 5 group to be treated with sterile saline and bleomycin A5 at 2 mg/kg, respectively.The mice were sacrificed on day 1, 3, 7, 14, and 21 after treatment.The inflammation score and collagen volume fraction ( CVF) of the lung tissue were obtained by HE and Masson staining .The total number and different types of cells in the bronchoalveolar lavage fluid ( BALF) were counted using the routine method .The mRNA expressions of collagens ⅠandⅢwere determined by real-time PCR, the content of hydroxyproline (HYP) assayed by the chloramine-T method, and the proportions of different monocyte subsets measured by flow cytometry . Results Compared with the saline control , the bleo-mycin A5 group showed significantly increases in the inflammation score at 3 and 7 days ( P<0 .01 ) , CVF at 14 and 21 days ( P<0.01), and the numbers of total cells and macrophages in BALF at 3-21 days, the count of neutrophils granulocytes at 1-3 days (P<0.01), The numbers of neutrophile granulocyles were significant higer than that in control groups on the 1st(9.086 ±1.268 vs 1.108 ±0.229), 3rd(5.551 ±0.511 vs 0.315 ±0.100) and 7th(8.093 ±0.922 vs 0.249 ±0.074)day.The mRNA expressions of collagens ⅠandⅢat 14 and 21 days (P<0.05), the content of HYP at 7-21 days (P<0.01), and the proportion of Ly6Chi mon-ocytes on day 1, which peaked on day 3 (P<0.01) and then decreased from day 14 to 21.The proportion of Ly6Chi monocytes was positively correlated with the inflammation score (P<0.000 1) and CVF of the lung tissue (P=0.001 3). Conclusion In the mouse model of bleomycin A5-induced pulmonary fibrosis, dynamic changes of circulating Ly6Chi and Ly6Clo monocyte subsets occurred in different pathophysiological stages .Compared with the pathological process of inflammatory infiltration , Ly6Chi circulating monocytes displayed a rapid response to tissue injury and inflammation .The increased proportion of Ly6Chi monocyte subsets might be closely re-lated with pulmonary inflammation and fibrosis .