Arsenic trioxide (As2O3) has positive therapeutic effects for a variety of malignant tumors,especially leukemia.The function of As2O3 is to inhabit proliferation and induce apoptosis of tumor cells.Here,our present study was to investigate whether As2O3 can serve as potential treatment for pulmonary fibrosis.3T3 fibroblasts were cultured and treated with 2,or 4 μmol/L As2O3 for 24h or 48h.Transmission electron microscopy (TEM) and Hoechst staining were used to characterize changes in cellular morphology of apoptosis.The methyl thiazolyl tetrazolium (MTT) assay was employed to assess cell proliferation,and cell cycle distribution was analyzed by flow cytometry (FCM).It was found that 3T3 fibroblasts treated with As2O3 exhibited morphological features consistent with early and mid-stage apoptosis,including changes in cell structure,chromatin margination,and chromatin condensation.As2O3 treatment significantly inhibited fibroblast proliferation in a time-and dose-dependent manner (P<0.01).FCM data revealed that the number of G0/G1 cells increased in a dose-dependent manner in As2O3-treated cells,coupled with a concomitant decrease in the number of cells in the G2/M phase.It is concluded that As2O3 inhibits cell proliferation of 3T3 fibroblasts by preventing cell cycle progression and promoting apoptosis.It might be employed as a potential chemical for the pulmonary fibrosis.