By the direct immunofluorescent technic, dengue antigen, human immunoglobulins, and beta 1C globulin were detectable on the surfaces of platelet suspensions from 48% of patients with dengue hemorrhagic fever. The percentages of positive-staining platelets were not related to the severity of thrombocytopenia, which was marked on the day after the patient developed shock or subsidence of fever. It is suggested that an immunologic mechanism is one of the factors associated with the thrombocytopenia caused by increased platelet destruction.