It is necessary in clinical to establish the effects of new substances for the primary and secondary prevention of thrombotic illnesses. We measured maximum aggregation after addition of collagen (F.C. 1.7 micrograms/ml) or ADP (F.C. 1.7 mumol/l) in 41 patients without drug treatment, 91 with ticlopidine, 22 with aspirin (ASA); 82-750 mg, 13 with ASA 81 mg, 20 with ticlopidine 100 mg and ASA 81 mg, 13 with cilostazol, 25 with flurbiprofen, 19 with nifedipine and evaluated the ex vivo effect of platelet antagonists. ASA inhibited remarkably collagen-induced platelet aggregation compared with other drugs, but there was significant (p less than 0.01 for 0.5 mumol/l ADP; p less than 0.02 for 1.0 mumol/l ADP) increase of primary aggregation induced by low dose of ADP in 14 healthy volunteers. While ticlopidine only significantly reduced ADP-induced platelet aggregation and we couldn't find dose dependency of ticlopidine (100-300 mg/day) on inhibitory effects of ADP or collagen-induced platelet aggregation. Our results indicate that the administration of one tablet of aspirin for pediatrics (aspirin 81 mg/tab) together with ticlopidine 100 mg is suitable for reduction of platelet aggregation.