Oil of cloves (OC) was found to be a potent inhibitor of platelet aggregation induced by arachidonic acid (AA), collagen and epinephrine; in this respect it was most effective against AA-induced aggregation. Inhibition of aggregation by OC seems to be mediated through a reduced formation of thromboxane as indicated by the following experimental evidence. (i) OC inhibited TxB2 formation in intact as well as lysed platelet preparations from added arachidonate, and (ii) it inhibited the formation of TxB2 from AA-labelled platelets after activation with Ca2+-ionophore A23187. The formation of lipoxygenase derived products was dependent on the concentration of OC used; at its lower concentration their amounts increased but this was found to be reversed at higher concentrations. At all concentrations thromboxane was decreased with a concomitant increase in unused AA.