Andrographolide, an active principle of the Chinese drug Andrographis paniculata, used for prevention and treatment of common cold in Scandinavia and known as an antiinflammatory, antiviral, antithrombotic, hypotensive and antiatherosclerotic drug, was investigated for its suggested influence on the biosynthesis of eicosanoids and the platelet-activating factor (PAF). Whereas in isolated human polymorph-nuclear leukocytes (PMNL) no influence on the biosynthesis was found, it could be shown that andrographolide inhibits PAF-induced human blood platelet aggregation in a dose dependent manner (IC50-5 microM). These results indicate that andrographolide has a mechanism of action different from that of non-steroidal antiinflammatory drugs (NSAID) and most likely associated with the cardiovascular and antithrombotic activity described of Andrographis paniculata.