Precipitin reaction between host organ extracts and antisera for Paragonimus and Clonorchis were studied to know whether the antigens from two species of trematodes have common characters with those of host organs. The brain, lung, heart, liver and muscle of rat and rabbit were extracted in 0.1 percent saline solution by freezing-thawing technique.

For immunization of rabbit, the extracts of Paragonimus and Clonorchis was emulsified with equal amount of Freund's adjuvant and 1.0 ml of the mixture was injected subcutaneously once every 10 days for 5 times, and 3 days after the last immunization, antisera were prepared. Precipitin bands by agar-gel double diffusion methods were examined between organ extracts of rabbit or rat and Paragonimus or Clonorchis antisera. The results could be summarized as follows:

1. Precipitin bands were appeared between extracts of liver, heart and muscle of rat and anti-Clonorchis rabbit sera but not appeared against brain and lung.

2. Precipitin bands were appeared between extracts of lung, muscle and liver of rat and anti-Paragonimus rabbit sera but not appeared against brain and heart.

3. Precipitin bands were appeared between extracts of liver and muscle of rabbit and anti-Clonorchis rabbit sera but not appeared against brain, heart and lung.

4. Precipitin bands were appeared between extracts of lung and muscle of rabbit and anti-Paragonimus rabbit sera but not appeared against liver.

5. The extracts of Paragonimus westermani showed precipitin bands against liver and brain of rat and liver of rabbit in agar-gel double diffusion.