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To explore the inhibition of juglone in Qinglongyi on A-549 cells in vitro, MTT assay was used. Laser confocal scanning microscope with AV-PI staining was used to observe apoptotic morphology. Changes of cell cycle are studied by flow cytometry analysis. The result of MTT assay showed that juglone had a marked growth inhibition in A-549 cells and the IC50 is respectively 3.4 times 10-5mol/L, 1.8 times 10-5mol/L and 2.6 times 10-6 mol/L after treatment for 24, 48 and 72 h by juglone. Through Laser confocal scanning microscope, we can see that juglone can induce the apoptosis, cells in the early stage of apoptosis were only stained by Annexin V-FITC, while necrotic cells and cells in the late stage of apoptosis were stained by both Annexin V-FITC and PI and appeared to be red inside and green outside. Cell cycle changes are analyzed by flow cytometry with cells at G1 phase significantly less than those of control and cells at G2 phase significantly more than those of control. This study has demonstrated for the first time that juglone from Qinglongyi can inhibit the proliferation of A-549 cells and block their cell cycle at the G2 phase, thus inducing apoptosis in these cells in distinct dose-dependent manner. But the specific mechanism responsible for the blocking of cell cycle and the induction of apoptosis has to await further research.