OBJECTIVE: To investigate the efficacy of Zhonglun'a-decoction-containing serum(ZHONGL-CS)on the in vitro apoptosis of fibroblast-like synoviocytes(FLS) from rats with collagen-induced arthritis(CIA) by investigating the Janus kinase/signal transducer and activator of transcription(JAK/STAT) signaling pathway.METHODS: A CIA rat model was established using bovine type Ⅱ collagen. FLS were isolated, cultured and identified. A cell counting kit-8 was used to detect cell activity. The half maximal inhibitory concentration(IC50) was calculated. Experimental subjects were divided into control, CIA,ZHONGL-CS, JAK2 inhibitor AG490, and ZHONGLCS with AG490 groups. The in vitro cell cycle and apoptosis rate were detected in FLS by flow cytometry. Bcl-2-associated X protein(Bax), B-cell lymphoma 2(Bcl-2), caspase-3, cyclin D1, phosphorylated JAK2, STAT1, and STAT3 protein expressions in FLS were examined by Western blotting. JAK2, STAT1 and STAT3 mRNA levels were examined by quantitative real-time polymerase chain reaction.RESULTS: Compared with the CIA group, FLS proliferation was inhibited, the FLS G0/G1 cell cycle was arrested, and the rate of FLS apoptosis was increased in the ZHONGL-CS group. In the ZHONGLCS group, the protein levels of Bcl-2 and cyclin D1 were reduced compared with the CIA group and the levels of Bax and caspase-3 in FLS were increased. In the ZHONGL-CS group, the expressions of JAK2, STAT1, and STAT3 mRNA and the levels of phosphorylated JAK2, STAT1, and STAT3 proteins were reduced.CONCLUSION: ZHONGL-CS may induce FLS apoptosis in CIA rats. Activation of the JAK/STAT signaling pathway was inhibited in FLS in vitro.