Indobufen is an inhibitor of platelet aggregation which acts by reversibly inhibiting the platelet cyclo-oxygenase enzyme. Indobufen under brand name Ibustrin is used for the following conditions: cerebrovascular insufficiency, atherosclerosis of peripheral and cerebral vessels, thrombophlebitis, deep vein thrombosis, and diabetes mellitus. In addition, this drug has been investigated in the phase II clinical trial for the prevention of thromboembolic events in patients with nonrheumatic atrial fibrillation. Racemic indobufen caused a dose-dependent inhibition of TXB2 and PGE2 production (IC50: 0.53 +/- 0.06 and 0.34 +/- 0.02 ug/ml, respectively). S-Indobufen was approximately 2-fold more potent than the racemate in inhibiting the synthesis of cyclooxygenase products. R-Indobufen affected the same enzyme but only at considerably higher concentrations (IC50: 53 +/- 8 ug/ml). Serum LTB4 concentrations were significantly reduced only at indobufen concentrations greater than 50 ug/ml. In conclusion, indobufen is a selective inhibitor of the cyclooxygenase activity of platelet PGG/H synthase in a concentration range corresponding to the therapeutic plasma levels in man. This inhibitory effect is largely due to the S isomer of the drug.