HPLC analysis of three Prunus species, Armeniacae Semen. Persicae Semen, and Mume Semen, showed that 24. 06 mg/g, 5. 79 mg/g and 3/ 10 mg/g of amygdalin in the MeOH extract and 3. 59 mg/g, 5. 41 mg/g and 13. 48 mg/g of benzaldehyde in the MeOH extract hydrolyzed with ${eta}-glucosidase$ were contained respectively. The MeOH extract of Mume Semen showed strong antibacterial activities against two species of bacteria (E. coli and B. subtilis) while that of Armeniacae Semen showed mild, and that of Persicae Semen showed no effect. Against A. niger, a strong antifungal activity was observed with Armeniacae Semen and only mild activities with Persicae Semen and Mume Semen. None oh the three inhibited the growth of S. cervisiae. The above results may possibly be suggestive of the correlationships between the contents of amygdalin and benzaldehyde in the test extracts with the antimicrobial potencies.