Summary Six prototypes of dengue viruses, West Nile and Sindbis viruses were cross-reacted in a rather standard type of immunoelectrophoresis test against four rabbit anti-dengue sera representing two closely related pairs as determined by more conventional serology. By this test D-1 and TH-Sman were readily differentiated, each possessing three or four antigens not possessed by the other. One antigen was common to both and to all other dengue protoypes and West Nile virus. A similar differentiation was demonstrated between D-2 and TH-36 and all other viruses employed. One D-2 antiserum demonstrated what may be a subgroup specific dengue antigen shared by all dengues but not by West Nile. The method gives promise of improved differentiation of arboviruses within an antigenic group.