The effects of teicoplanin on adenosine-diphosphate (ADP)-induced human platelet aggregation in vitro and on both ADP- and ristocetin-induced human platelet aggregation ex vivo were investigated. In the in vitro study carried out on platelets from 7 healthy volunteers, teicoplanin had no effect on platelet function even at a concentration (1 mg/ml) 10 times higher than the peak level found in the in vivo state, but at the highest concentration (10 mg/ml), which is 100 times higher than that reached in vivo, it inhibited ADP-induced platelet aggregation. In the ex vivo studies carried out in 10 healthy volunteers, teicoplanin, following single intravenous doses of 400 mg and 800 mg, did not produce any effect on platelet function up to 6 hours after administration. After 12 hours, teicoplanin, when given at 800 mg i.v., reduced ADP-induced platelet aggregation.