A method is described for studying platelet function in human whole blood immediately after venepuncture in order to evaluate the antithrombotic potential of new pharmacological agents. In this method, platelet aggregation is quantified by measuring the fall in single platelet count, by using a whole blood platelet counter. We have investigated the platelet aggregation inhibitory effects of the new positive inotropic agents pimobendan and UD CG 212 (reported to be Ca++ sensitisers and phosphodiesterase inhibitors), alone and in combination with dipyridamole. Venous blood was drawn directly into prewarmed (37 degrees C) plastic syringes containing anticoagulants (3.2% trisodium citrate solution) plus a platelet aggregation inhibitor. Spontaneous platelet aggregation (SPA) was studied by roller mixing aliquots of blood in the collecting syringes for 6 min at 37 degrees C. Collagen induced platelet aggregation was studied by incubating aliquots of blood with 1 microgram/ml collagen on a shaking water bath for 3 min. In the absence of an inhibitor, there was a 50% fall in single platelet count due to SPA and a 65% fall was induced by collagen. Both SPA and collagen induced aggregation responses were inhibited by pimobendan (0.5-10 µM) and UD CG 212 (0.5-10 microM), in a dose dependent manner. A combination of 10 microM dipyridamole with 2 microM pimobendan or UD CG 212 was markedly a more effective inhibitor of platelet aggregation than a high dose of either inhibitor alone. It is suggested that the present method is simple and rapid, with minimal sample processing, and therefore the results may be protected from serious artifacts.This method may be suitable for evaluating the antithrombotic potential of pharmacological agents in vitro or ex vivo.