Sera from 282 equines from Tandil country and surroundings were investigated searching for hemagglutination inhibition (HI), Complement fixation (CF), and Neutralizing (NT) antibodies against three flavivirus:Ilheus, St. Louis Encephalitis, and Yellow Fever from the Togaviridae family. Sera were collected between 3-20-79 and 11-25-80 from 10 different places in Tandil and Ayacucho countries. Animals ranged from 45 days to 27 years old. Forty nine of them reacted with one or more flavivirus by HI and/or CF tes representing a prevalence of 17.4% for this antigenic complex. Twenty four of them neutralized ILH, 6 SLE and from them 2 neutralized both viruses. Twenty one cannot be endorsed to ILH or SLE but to some other/s flavivirus present in the environment but not present in the test since they did not neutralize any of them. Other 12 sera with no HI or CF antibodies for any of the three viruses did not neutralize ILH or SLE. Results showed ILH and SLE viruses, both isolated in man during previous years studied in Argentina, to be endemic in the area and to be naturally infecting horses, perhaps without producing detectable disease, since no epizootics due to SLE or ILH viruses were reported on the horse population investigated.