Ticlopidine (T) and aspirin (ASA) are two antiplatelet drugs both capable of prolonging bleeding time (BT), with a different mechanism of action. A synergism in BT prolongation has been reported and is currently considered an argument for not recommending their combination. However, a profound suppression of platelet function might be a desirable counterpart of a marked prolongation of BT, with a possible use in selected clinical situations. We therefore studied ex vivo platelet function (aggregation by ADP 0.5-1-2.5 microM; adrenaline 0.75-2.5 microM; collagen 1.5-150 micrograms/ml; arachidonic acid 1 mM; PAF 1 microM; adrenaline 0.17 microM + ADP 0.62 microM; serum thromboxane [( TX]B2 generation) and BT (Mielke) in 6 patients with stable coronary artery disease receiving such combination. Patients underwent sequential laboratory evaluations at baseline, after 7 days of T 250 mg b.i.d., before and after the intravenous administration of ASA 500 mg, respectively, and, finally, after a minimum of 7 days of sole ASA oral administration (50 mg/day). The experimental design, therefore, allowed a comparison of T and ASA effects (2nd and 4th evaluation), and an assessment of the combination effect (3rd evaluation). Platelet aggregation in response to all doses of ADP was depressed more by T than by ASA. Conversely, responses to adrenaline, and arachidonate were affected more by ASA than by T. For all other agents, differences were not significant. T + ASA combination was more effective (p less than 0.05) than either treatment alone in depressing responses to high-dose collagen (% over control, mean +/- SEM: T: 95 +/- 3; ASA: 96 +/- 5; T + ASA: 89 +/- 4).(ABSTRACT TRUNCATED AT 250 WORDS)