ObjectiveTransforming growth factor-β₁ （TGF-β₁） was used to stimulate human fetal lung fibroblast 1 （HFL1） for simulating the pathological process of idiopathic pulmonary fibrosis （IPF） and thereby the effects and mechanism of medicated serum of Bupleuri Radix against IPF were investigated. MethodTGF-β₁ （10 μg·L^-1） was employed to stimulate HFL1， and cells were treated with medicated serum of Bupleuri Radix （5%， 10%， 15%， 20%） for 24 h. Then cell proliferation rate was determined with cell counting kit-8 （CCK-8）. Subsequently， cells were classified into the control group （20% blank serum）， TGF-β₁ group （20% blank serum and 10 μg·L^-1 TGF-β₁）， TGF-β₁ + medicated serum of Bupleuri Radix group （5% blank serum， 15% medicated serum， and 10 μg·L^-1 TGF-β₁）， and TGF-β₁ + SIS3 group （3 μmol·L^-1 SIS3， 20% blank serum， 10 μg·L^-1 TGF-β₁）. Based on in situ end labeling （TUNEL） staining， the apoptosis rate was examined， and mRNA expression of apoptosis-related proteins B-cell lymphoma 2 （Bcl-2）， Bcl-2 associated X protein （Bax） and myofibroblast marker α-smooth muscle actin （α-SMA） was detected by Real-time fluorescence quantitative polymerase chain reaction （Real-time PCR）. The protein expression of α-SMA， Ras homolog enriched in brain （Rheb）， and phosphorylated （p）-Smad3 was determined by immunofluorescence. Expression of Rheb， p-Smad3， and Smad3 was examined by Western blot. ResultThe cell proliferation rate of TGF-β₁ group increased compared with that of the control group （P<0.05）. The cell proliferation rate of TGF+15% medicated serum of Bupleuri Radix group and TGF+20% medicated serum of Bupleuri Radix group decreased compared with that of the TGF-β₁ group （P<0.01）. Compared with the control group， TGF-β₁ group showed decrease in apoptosis rate， increase in mRNA expression of Bcl-2 and α-SMA， reduction in Bax mRNA expression， and rise of α-SMA and Rheb protein expression and p-Smad3 level （P<0.05）. Compared with TGF-β₁ group， TGF-β₁ + medicated serum of Bupleuri Radix group and TGF-β₁ + SIS3 group demonstrated high apoptosis rate， low Bcl-2 and α-SMA mRNA expression， high Bax mRNA expression， and low α-SMA and Rheb protein expression and p-Smad3 level （P<0.05）. ConclusionMedicated serum of Bupleuri Radix can inhibit TGF-β₁-induced HFL1 proliferation and fibroblast-myofibroblast transition and promote fibroblast apoptosis by regulating the Smad3/Rheb axis.