The in vitro effects of N-3-(1-benzyl-cycloheptyloxy)-propyl-N,N-dimethylammonium-hydrogenfumarate (bencyclan) on clotting, fibrinolytic and platelet function test were investigated by adding the drug to normal human plasma. An anticoagulant activity, mainly of an antithromboplastin nature (directed against later stages of intrinsic thromboplastin formation and against tissue thromboplastin), was observed, while thrombin phase was unaffected. No effect was found in the fibrinolytic system tested (euglobulin lysis, UK-activated fibrinolysis, "hanging clot" method). The drug, although capable of aggregating platelets by itself at very high concentrations, showed a striking inhibitory effect, over a wide range of concentrations, both on platelet aggregation induced by ADP, epinephrine or collagen and on platelet adhesiveness to glass or collagen. Clot retraction was also clearly inhibited. PF3 availability was influenced with a peculiar two-phase behaviour dose-dependently. High concentrations showed a promoting action, while the lower were obviously inhibitory. It is suggested that the effects on platelet function may be due to an influence of the drug on cell membrane.