The anticoagulant used for blood collection may modify platelet aggregation. In platelet-rich citrated plasma, adrenaline induced biphasic aggregation accompanied by 14C-serotonin release, whereas in platelet-rich hirudin plasma it induced weak primary aggregation. In plasma from blood anticoagulated simultaneously with hirudin and citrate, adrenaline caused biphasic aggregation as it was observed in citrated plasma. Qualitative differences in platelet reaction in hirudin citrated plasma were found to be dependent on calcium ion concentration. Therefore, the deprivation of calcium ions by citrate is considered the cause of the adrenaline effect in citrated plasma.