Hlroshl IMAGAWA,1) Yoshio FUKUNAGA1) and Masanobu KAMADA1) Neutralization test (NT) was conducted for detection of neutralizing antibody against calf diarrheal coronavirus (CDCV) for application to serological survey among horses . The plaque-reduction neutralization test (PRNT) and the micro-titer neutralization test (MTNT) were examined with Kakegawa strain of CDCV and HmLu-1 (humstar lung) cells. The specific serum neutralizing antibody detected by PRNT and MTNT developed at the same rate

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... h the PRNT titer being higher than the MTNT titer in the sera of two horses experimentally inoculated with CDCV. To know the difference of the antibody titers detected by both neutralization tests, both serum neutralization tests were performed in serum samples of 100 racehorses. Consequently, 40 samples were negative (less than 5) in both tests, 19 samples were positive (5 or more than 5) by PRNT and negative by MTNT and remaining 41 samples were positive for both tests. The titers detected by both tests were closely correlated with each other, although neutralizing antibody titers detected by PRNT were higher in the order of 2 to 16-fold than those detected by MTNT. According to the serological survey by MTNT, 3 to 60% horses in rearing farms and 520 horses in breeding farms possessed the neutralizing antibody against CDCV, respectively. From these results, it was demonstrated that a viral agent antigenically related with CDCV existed widely among horses in ,Japan.