Objective: To evaluate the synergistic effect of Radix Astragalus A6 (A6) and acyclovir (ACV) in antagonizing herpes simplex virus type Ⅰ (HSV1). Methods: The synergistic effect was measured by competitive polymerase chain reaction (PCR) and compared with that measured by traditional cytopathic effect (CPE) inhibition method. Results: The minimum inhibition concentration of A6, ACV and A6 + ACV measured by PCR was 1. 88 mg/ml, 3. 37 μg/ml and 0. 47 mg/ml + 0. 84 μg/ml respectively, while measured by CPE, it was 6. 25 mg/ml, 50 μg/ml and 0. 94 mg/ml + 12. 5 μg/ml respectively. The fractional inhibitory combined indexes were all less than 0.5, which is an indication of obvious co-synergistic effect. Conclusion: The quantitative PCR is an effective method of screening antiviral drugs. The inhibitory action of A6 and ACV on HSV1 revealed mainly at the replicative stage of viral proliferation cycle.Original article on CJIM(Chin) 1998; 18(4): 233 Objective: To evaluate the synergistic effect of Radix Astragalus A6 (A6) and acyclovir (ACV) in antagonizing herpes simplex virus type I (HSV1). Methods: The synergistic effect was measured by competitive polymerase chain reaction (PCR) and compared with that measured Results: The minimum inhibition concentration of A6, ACV and A6 + ACV measured by PCR was 1. 88 mg / ml, 3. 37 μg / ml and 0. 47 mg / ml + 0 . 84 μg / ml respectively while while by CPE, it was 6. 25 mg / ml, 50 μg / ml and 0. 94 mg / ml + 12.5 μg / ml respectively. The fractional inhibitory combination indexes were all less than 0.5: which is an indication of obvious co-synergistic effect. Conclusion: The quantitative PCR is an effective method of screening antiviral drugs. The inhibitory action of A6 and ACV on HSV1 revealed mainly at the replicative stage of viral proliferation cycle. CJIM (Chin) 1998; 18 (4): 233