Charcoal haemoperfusion is used in the treatment of severe drug overdosage, and is potentially useful in fulminant hepatic failure and chronic uraemia. Haemo-compatibility of early devices has improved, but platelet loss and occasional associated haemorrhage still occur. During a double blind cross-over study, in 8 healthy male volunteers, aspirin 600 mg/day, sulphinpyrazone 800 mg/day, or a combination of both or placebo were administered for 2 days at 14 day or longer intervals. The effects of these agents were measured with reference to 1) forearm bleeding times using a spring loaded device, 2) platelet adsorption during 2 hrs haemoperfusion of heparinized blood through 2% acrylic hydrogel coated or uncoated activated charcoal in vitro, and 3) platelet retention from citrated blood on PT150 and PT250 cuprophan dialysis membranes in vitro. Aspirin and aspirin in combination with sulphinpyrazone prolonged bleeding time, but sulphinpyrazone alone had no significant effect on bleeding time. Aspirin alone, sulphinpyrazone alone, and their combination reduced platelet adsorption on coated activated charcoal, and platelet retention on cuprophan membranes. Aspirin, and aspirin in combination with sulphinpyrazone, reduce platelet adsorption on uncoated charcoal, while sulphinpyrazone alone had no significant effect. The results indicate that 2 days treatment with sulphinpyrazone or aspirin produce significant antiplatelet action and that the effects of sulphinpyrazone on platelet function should be explored in relation to clinical haemoperfusion and haemodialysis.