Antibodies against dengue viral proteins were demonstrated in sera from dengue-infected patients by polyacrylamide gel electrophoresis, Western blotting, and enzyme immunoassay. Primary dengue cases showed low titers of IgG class antibodies to envelope (E) proteins and two non-structural proteins, NS3 and NS5, in sera collected during the convalescent phase. Secondary dengue- infected patients always demonstrated IgG antibodies to E proteins in sera collected during the acute phase, and high titers of IgG antibodies to many other proteins, including NS1, NS3, NS5, and C proteins in sera collected during the convalescent phase. Appearance of antibodies to E, NS3, and NS5 could be detected within five days after the onset of fever. These three dengue viral proteins and their corresponding antibodies may be involved in the immunopathologic mechanism underlying this disease. For diagnostic purposes, identifying the non-structural proteins such as NS3 and NS5 may be the best means for early confirmation of the disease.