Abstract: In order to investigate the significance of the co-stimulatory molecules CD80(B7-1)/CD86(B7-2) in abortion and the anti-abortion mechanisms of Scutellaria baicalensis Georgi and its components, mifepristone (90 μg per mouse) was injected subcutaneously (sc) to induce abortion in BALB/c mice. The number of CD80+ and CD86+ cells in the spleen were assayed by flow cytometry. The results revealed that the CD80+ cells significantly elevated, however, the CD86+ cells significantly decreased in group B (the group treated with RU486 ) in comparison with group A (the control group). The decoction of Scutellaria baicalensis Georgi, and its components (baicalin or baicalein) given orally to the pregnant mice prior to RU486 treatment group significantly inhibited the abortifacient effect of RU486, and its CD80+ cells number significantly decreased, the CD86+ cells number significantly elevated in comparison with the group B. These results suggested that RU486 inducing abortion associated with the CD80+ and CD86+ cells number in the spleen, and that the antiabortion effects of the Scutellaria baicalensis Georgii, its componetnts (baicalin or baicalein) are realized through modulation of the number of CD80+ and CD86+ cells in the spleen.