Treating the rabbits with each of the blood sera of the domestic ducks, wild ducks and domestic geese belonging to Anatidae, the blood-relationship amongst these stocks were examined by means of precipitin reaction, and the results are as follows: 1. The Japanese duck is distinguishable from the Chinese goose with the corresponding antisera. 2. The Japanese duck and the Muscovy duck, are not distinguishable from each other with the corresponding specialized antisera. 3. The Japanese duck can not be distinguished from the wild ducks with the corresponding antisera. By means of the corresponding specialized antisera, however, it can be clearly distinguished from Eunetta, Marecca as well as Nettion crecca, and the Muscovy duck from Anas boschas. 4. The Japanese duck can be distinguished from its wild form (Anas boschas) with the specialized antiserum for the former, but not with that for the latter. In conclusion, the writer desires to express his sincere thanks to Prof. M. ISHIHARA under whose direction this work was done, and to Prof. T. KUBO for selecting the material. He is also indebted to Prof. M. ITAGAKI for kind advice, and to Asst. Prof. T. MISAO for much valuable help.