Autologous skin tests with DOC-solubilized-tumor extracts including 20 cases of colon cancer and 25 cases of gastric cancer revealed the following: 1. There were 9 positive reactions and 11 negative reactions in the colon cancer cases. On the other hand, there were 16 positive reactions and 9 negative reactions in the gastric cancer cases. 2. In gastric cancer cases, the time span of autologous skin tests revealed that curable cases with positive skin reactions had negative reactions after operation and uncurable cases remained negative after operation. 3. There was no relationship of the clinical stage and nonspecific skin test (PPD skin test) to the autologous skin test.