Normal adult female mice were inoculated with the virus of foot-and-mouth disease, and sera obtained from these animals at intervals up to 3 months after inoculation were tested for their neutralizing capacities. Neutralizing antibody was detected as early as 3 days after inoculation and increased gradually until a maximum was reached at 20-30 days. The antibody persisted at a high level throughout the remainder of the 84-day period.