Uncommon serotypes were tested for their ability to induce aggregation in platelet-rich plasma. Both uncommon and common serotypes induced platelet aggregation in rabbit platelet-rich plasma, but serotonin release was higher with the uncommon serotypes. Aggregation at ATP release varied between uncommon and common serotypes. With human platelet-rich plasma, only uncommon serotypes showed aggregation. Differences in serotype selectivity and the rate of aggregation were noted among platelet donors. About half of the patients with Behcet's disease and 30% of health controls showed platelet aggregation. Plasma from non-responder patients with Behcet's disease inhibited aggregation of healthy responder platelets within 20 min. Thus selective binding of uncommon serotypes of Strep. sanguis to plateletes might cause the vasculitis in Behcet's disease.