The anti-thrombic properties of the oriental herbal medicine Daejowhan(DJW, 大造丸) which consists of 11 kinds of herbs (indicated as ratio) of Rehmanniae Radix 24%, Hominis Placenta 5%, Testudinis Carapax 9%, Eucommiae Cortex 9%, Asparagi Radix 9%, Phellodendri Cortex 9%, Achyranthis Radix 7%, Liriopis Tuber 7%, Angelicae Sinensis Radix 7%, Ginseng Radix 5% and Schizandrae Fructus 3% were investigated. The water extracts from DJW inhibited Platelet-activating factor(PAF) induced platelet aggregation. DJW was extracted with methanol and further fractionated by ethylacetate. A 70% methanol extract showed a strong inhibition against PAF-induced aggregation in vitro and in vivo assays. The ethylacetate soluble fraction was shown to have inhibitory effect on PAF-induced platelet aggregation in vitro assay. The ethylacetate soluble fraction specially protected against the lethality of PAF, while verapamil did not afford any protection. These results indicate that the water extracts and alcoholic-fractions inhibit the action of PAF in vivo by an antagonistic effect on PAF, so that it may be useful in treating disorders caused by PAF, such as acute allergy, inflammation, asthma, gastrointestinal ulceration, toxic shock and so forth. DJW was investigated regarding its assumed anti-thrombic action on human platelets which was deduced from its ability to suppress Arachidonic acid(AA)-induced aggregation, exocytosis of ATP, and inhibition of Cyclooxygenase(COX) and Thromboxane synthase(TXS) activity. The latter two effects were estimated from the generation of Prostaglandin $E_2(PGE_2)$ and Thromboxane $A_2(TXA_2)$ respectively. Exogenously applied AA ($100{mu}mol/{ell}$) provoked a $89\%$ aggregation of platelets, the release of 14 pmol ATP, and the formation of either 225 pg $TXA_2$ or 45 pg $PGE_2$, each parameter being related to 106 platelets. An application of DJW 5 min before AA dose-dependently diminished aggregation, ATP-release and the synthesis of $TXA_2$ and $PGE_2$ with $IC_{50}$ values of 74, 108, 65, $72{mu}g/m{ell}$, respectively. The similarity of the $IC_{50}$ values suggest an inhibition of COX by DJW as primary target, thus suppressing the generation of $TXA_2$ which induces aggregation of platelets and exocytosis of ATP by its binding on $TXA_2$-receptors.