A total of 1024 sera obtained from horses were divided into 4 groups and examined for serum neutralizing (SN) antibody against equine rhinovirus serotype 1 (NM 11 strain) to clarify the spread of this serotype over a period from 1974 to 1977 in Japan. The positive rate of SN antibody was 16.5% in group 1 consisting of 200 yearlings. It was 85.5% in group 3 consisting of 2 and 3-year-old horses belonging to the Tokyo Racecourse, Japan Racing Association (JRA), and 88.0% in group 4 consisting of horses more than 4 years old belonging to the Equestrian Park, JRA. On the other hand, the same survey was conducted on the Utsunomiya Rearing Farm, JRA in every rearing period for 1974 to 1977. The positive rate of SN antibody in 55 horses was 24.1% at 1 to 2 years of age when they were introduced into this farm, and increased significantly to 100% in the terminal period of rearing. Of the 55 horses, 11 had no detectable SN antibody during the rearing period in any year. Of them, 4 horses acquired SN antibody immediately after they were introduced into the racecourse. These results suggested that equine rhinovirus serotype 1 might have been distributed widely among horses in Japan. Horses 1 to 2 years old in the course of rearing were observed to be most susceptible to this virus infection.