The antigenic interrelationship between 52 Madagascan West Nile isolates and two prototype viruses (Eg101 and G2266) was assessed by an immunofluorescent technique using monoclonal antibodies. This analysis enabled us to define 5 groups of variants, 4 of which were closer to the Egyptian strain (Eg101) than to the Indian prototype (G2266). Groups II and V were dominant whereas strains of groups I and IV were less numerous. One strain belonging to group III was antigenically similar to the Indian strain. Antigenic variations were observed among viruses isolated from man, birds and different mosquito genera. Geographic variations were also observed. Exchanges exist between Madagascar and the African continent by means of migratory birds which seem to be instrumental in disseminating the virus and introducing the antigenic variants.