Young, N. A. and K. M. Johnson (Middle America Research Unit, Box 2011, Balboa Heights, Canal Zone). Antigenic variants of Venezuelan equine encephalitis virus: Their geographic distribution and epidemiologic significance.Amer. J. Epid.,1969, 89: 286–307.—Antigens from 124 virus strains of the VEE complex, supplied by virtually all laboratories and countries reporting isolation of these agents, were tested by a kinetic hemagglutination-inhibition technique using Proechimys semispinosus (spiny rat) immune sera. This assay system was reproducibly capable of detecting two- to fourfold differences In antigen units inhibited between virus strains not otherwise measurable by the standard HI technique. Observed antigenic relationships were stable upon passage and not related to the donor host of virus strains. Place of origin and to a lesser extent time were important determinants of antigenic variation. Thus it was possible to differentiate viruses from Florida, Mexico, Panamá, Colombia, Venezuela, the island of Trinidad, and Brazil. The significance of these geographic variants is discussed with regard to current concepts of the ecology and epidemiology of viruses of the VEE complex. The data also provide new insight into past epidemics.