Abstract: The Chinese drug, Ya-tan-tsu (fruit of Brucea javanica L., Family Simarubaceae), was found to be effective in chick's malaria, but the clinical reports in recent years showed somewhat contradictory results. It is found by chick's malaria test that samples obtained from different market sources vary in therapeutic effectiveness. This phenomenon may explain why contradictory clinical results were usually obtained. It is therefore suggested that tablets made from biologically-standardized extract should be used for clinical trials. The active principle of Ya-tan-tzu appeared to be quite stable, because its extract has been found to be still active while prepared by drying its decoction and stored as long as 17 months.