Representative members of four arbovirus taxons were studied by immune electron microscopy (IEM) in an attempt to determine whether the technique can be applied to the specific identification of arboviruses. Alphavirus, flavivirus, bunyavirus, and rhabdovirus antigens were prepared by propagation in tissue culture. The viruses were used to titrate homologous antibody and test heterologous antibody in the IEM test. Specific clumping together with visualization of antibody attachment was observed for all homologous systems studied. For the alphaviruses, titers of antibody, as determined by IEM, were comparable with the titers obtained by hemagglutination inhibition. Alphavirus cross-reactions were observed at low antibody dilutions by IEM. However, further dilution of antibody resulted in specific clumping only of homologous virus, allowing specific identification of alphavirus unknowns. For all viruses studied, positive IEM reactions were obtained with homologous grouping ascitic fluids but not with heterologous grouping fluids.