The rapid development in the knowledge about the synthesis and metabolism of the various products derived from arachidonic acid is a continuous chal- lenge for those who intend to measure these substances in biological fluids. The confusion which still exists about their possible role in some physiolo- gical and pathological situations is partly due to the methodological pro- blems related to the estimation of these metabol ites in subnanogram quanti- ties. This concern has inspired the Commission of the European Communities to sponsor a first workshop on the "Clinical Application of Assay...