The appearance and persistence of IgM antibodies were studied by MAC-ELISA in 926 confirmed cases of dengue virus type 1 infection. Assays performed on acute and convalescent patient sera revealed that IgM antibodies appeared during the early phase of disease (day 2) and persisted for three months after onset. MAC-ELISA proved to be a valuable early diagnosis test when compared with haemagglutination inhibition.