SummaryHorses immunized by injection of live Edmonston measles virus in Freund's Arlacelmineral oil adjuvant produced measles antisera of high neutralizing antibody content and in large amount. The response to aqueous measles vaccine was very poor. The antiserum was devoid of bothersome nonspecific substances and was not toxic for human cells in tissue culture. The economic and scientific utility of horses for preparing anti-sera against viruses was discussed.