MCI-9042, a potent inhibitor of 5HT2 receptor, was used to examine its effects on serotonin induced-aggregation of platelets obtained from patients with type 2 diabetes mellitus (DM). The extent of platelet aggregation induced by serotonin increased in DM patients without retinopathy, but the increase was further significant in DM patients with retinopathy. MCI-9042 as well as ketanserin, inhibited significantly serotonin induced-aggregation of platelets obtained from DM patients with retinopathy. MCI-9042 dose dependently inhibited platelet aggregation induced by serotonin and collagen. These results suggest that serotonin which may be released from platelets of DM patients may activate platelets together with collagen exposed on atherosclerotic endothelium and that MCI-9042 may be inhibitory to enhanced platelet aggregability under these conditions.