Henderson, B. E., W. A. Chappell, J. G. Johnston, Jr. and W. D. Sudia (CDC, Atlanta, Ga. 30333). Experimental Infection of horses with three strains of Venezuelan equine encephalomyelitis virus. I. Clinical and virological studies. Amer J Epidem 93: 194–205, 1971.—Clinical effect, viremias and antibody responses were compared in horses inoculated with the following strains of Venezuelan equine encephalomyelitis (VEE) virus: Fe3-7C, isolated from Culex (Melano-conion) spp. mosquitoes collected In 1963 in the Florida Everglades; TC-83, the Fort Detrick live, attenuated vaccine strain; and GJ9-1BJ, an isolate from Psorophora confinnis mosquitoes collected in Guatemala during the 1969 epidemic of VEE. Six horses were inoculated with Fe3-7C, and, except for a febrile response of less than 24 hours' duration in one horse, no clinical responses were noted. Low-level viremias were detected in two horses. Six horses were also inoculated with TC-83. Five had a febrile response, and two of these horses were ill for two to three days. All of the TC-83 inoculated horses had detectable viremias, but the titers were low, i.e., 10 SMICLD50/0.02 ml. Of four horses inoculated with GJ9-1BJ, all had a marked febrile response and high-titered viremias (3.5–5.5 log10 SMICLD50/0.02 ml) and one died. The 12 horses previously inoculated with Fe3-7C and TC-83 had neither clinical response nor viremia when challenged on the 34th and 28th days, respectively, with GJ9-1BJ. The pattern of development and persistence of hemagglutination inhibiting, complement fixing and neutralizing antibodies indicates that Fe3-7C antibodies are readily distinguishable from TC-83 and GJ9-1BJ antibodies, although there is cross-neutralization between all three virus strains.