Summary A simple immunoelectrophoresis (IE) method has been shown to be useful and rapid in the antigenic study of arboviruses. Initial evaluation was made by testing viruses of groups A, B, Tacaribe and vesicular stomatitis in their homologous systems by the complement-fixation, hemagglutination-inhibition and IE methods. Seven strains of Sindbis virus were readily differentiated in IE studies, but dengue type 1 virus could not be distinguished from strain TH-Sman nor dengue type 2 from strain TH-36. Results of additional IE tests with a few samples of convalescent and apparently normal human sera suggest that this method may also be useful as a diagnostic procedure in the study of arboviruses.