A new class of drugs has appeared alongside classical anti-thrombotic agents such as heparin and oral anticoagulants, characterised by their ability to modify the behaviour of platelets: "anti-platelet" agents. This article reviews the platelet actions, pharmacokinetics, conditions of use and side effects of the four chief agents available: acetylsalicylic acid (ASA), sulfinpyrazone, dipyridamole and ticlopidine. The mode of action of the first of them is that best studied. ASA opposes the conversion of arachidonic acid to prostaglandins and thromboxane, by the irreversible acetylation of cyclo-oxygenase. Nevertheless, major therapeutic trials involving ASA have yielded only poor results. There are at least two possible explanations for this state of affairs: --aggregation may occur even when thromboxane is blocked, in particular in response to thrombin; --ASA has been used at doses also capable of inhibiting the formation by the vascular wall of an anti-aggregant prostaglandin, PGI2. Current efforts by pharmacologists which should result in better adapted and hence more effective anti-thrombotic methods, are essentially concerned with the following points: --to understand why sulfinpyrazone, which in principle has the same mode of action as ASA, seems sometimes more active and sometimes less active than the latter according to whether coronary or cerebrovascular accidents are involved; --to propose a rational prescription programme for ASA, in such a way that it inhibits only little, and for as short a time as possible, the production of PGI2 (e.g. 200 mg every three days): --to perfect more active combinations; --synthesis of new substances, e.g. thromboxane synthetase inhibitors, or stable analogues of PGI2. The reasons which suggest that such substances could be used more beneficially in man are expanded.