The usefulness of IgM antibody capture enzyme-linked immunosorbent assay (ELISA-IgM) for the diagnosis of dengue was studied using several groups of sera. An attempt was made to detect IgM against each dengue serotype. Of 76 paired sera from patients with dengue type 4 infection, the diagnosis was confirmed by ELISA-IgM in 74: in 47 out of 49 primary dengue infections and in all 27 secondary dengue infections. It was possible to detect IgM in 15 of the acute sera of primary infections and in 16 of the acute sera of secondary infections. In 20 patients from whom only single serum with high haemagglutination-inhibition titre (greater than or equal to 640) was available, 18 were found positive for dengue IgM. Dengue infection was also confirmed in 10patients with dengue type 1 and in 10 patients with dengue type 2. IgM was detected in 1 of the acute sera of patients with either dengue type 1 or dengue type 2 infection. In all cases in which IgM was detected, a positive response was obtained for the homologous antigen. ELISA-IgM on sequential serum samples from 7 proven cases of dengue type 4 revealed that antibody titres greater than or equal to 400 are reached early in the second week of the illness and persist for 60 days or more at a low level (titre less than 400). An attempt was made to obtain a type-specific presumptive diagnosis by analysing the relative titre
against each of the 4 antigens. It was possible to make a diagnosis of dengue type 4 in 85% and 52% of IgM-positive responses in primary and secondary infections (p less than 0.01), respectively, using paired sera from patients with known dengue type 4 infection. ELISA-IgM was shown to be of interest in the diagnosis and surveillance of dengue in French Polynesia where dengue viruses are, presently, the only known flavivirus.