Our investigations have demonstrated on 10 volunteers receiving either 500 mg or 100 mg acetylsalicylic acid (ASA) that a low collagen concentration (1 microgram/ml) can best detect the aggregation defect caused by ASA. With the impedance aggregometry the mean inhibition reaches 82% and 52% with 500 mg and 100 mg ASA, respectively. Collagen at higher concentration (3 micrograms/ml) as well as ADP 10 and 25 mumol/l are less sensitive, less than 25% inhibition was recorded. These results suggest that a 1 microgram/ml concentration of collagen is adequate for the control of the ASA effect up to 6 days after intake of 100 mg. Furthermore, the von Willebrand factor (vWF) dependent platelet aggregation induced by 0.6 and 1.0 mg/ml ristocetin was clearly diminished after ASA. Therefore, a ristocetin screening test in whole blood for vWF disorder is possibly distorted when the test is performed within 6 days from ASA administration.