From August to December 1987, 937 sera from dairy cows in Saitama and Aichi Prefectures were tested for hemagglutination inhibition (HI) antibody to Japanese encephalitis virus (JEV). Monthly positivity rates as well as antibody titers were the highest in September and the lowest in December.Nineteen of 79(24%) from Saitama and 12 to 14(86%) from Aichi showing antibody titers of 1:40 or higher were suggested to have been very secently infected becouse of 2-mercaptoethonol (2ME) sensitivety of antiblody.