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Forsythia suspensa extracts, isolated from the air-dried fruit of Forsythia suspensa, is a common traditional Chinese medicine recorded in Chinese Pharmacopeia. To evaluate the in vitro antibacterial activity of Forsythia suspensa extract, the minimum inhibitory concentration (MIC) was determined using a microtitre assay. For in vivo investigation, 50 Kunming mice were used, groups 1 to 4 (10 per group) were inoculated intraperitoneally with 1 ml of S. suis (LD50 CFU/ml) and treated orally 3 times per day for 7 days with: group 1,group 2, group 3 and group 4. Another group (group 5) of 10 mice were control group. Serial dilutions of the lung homogenates were counted after an 18 hour incubation at 37°C.Ethanol extracts of Forsythia suspensa showed significant bacteriostatic activity against all tested strains of S. suis with MICs all greater than 1.562 mg/ml, especially the amoxicillin-resistant strains. When tested in vivo, a 1:4 mixture of amoxicillin and Forsythia suspense extract had the greatest protective effect on mice against S. suis, followed by Forsythia suspensa extract alone.