SummaryThe methods available to the clinical pharmacologist for evaluation of anti-aggregating compounds in man are briefly presented. A distinction is made betweenex vivomethods (in vitro evaluation of the modified platelet adhesion or aggregation following drug administration) and strictlyin vivotechniques (the bleeding time and the platelet survival time). The effects of dipyridamole, sulfinpyrazone, acetylsalicylic acid, clofibrate and dextrans on these indices of platelet behaviour are briefly described. It is emphasized that platelet function is altered in a very similar fashion following administration of either dipyridamole or sulfinpyrazone; acetylsalicylic acid however modifies platelet behaviour in a completely different manner. A possible synergism between these two types of compounds has been proposed.