As a link in the studies on biological activity of lymphocytes in the mice transplanted with Ehrlich ascites cancer and in cancer patients the correlation between machrophage migration inhibitory activity (MIF-activity) and the progress of cancer was investigated, and the results are presented briefly as follows. 1. MIF activity in the mouse transplanted s.c. with 5x10(6) Ehrlich cancer cells first appears markedly in the regional lymph nodes and the MIF activity disappears about 10 days after the cancer transplantation. 2. In three cases (8.6%) out of 36 having benign diseases MIF activity is positive. 3. In measuring the MIF activity of peripheral blood lymphocytes of 71 gastric cancer patients before operation, those showing positive or pseudo-positive activity of MIF are 24 cases, being 33.9% of the total. MIF activity of peripheral blood lymphocytes before operationin the cases of Stage II is 41.7% , in those of Stage III it is 29.0% and in those of Stage IV it is 15.4% , indicating that the positive rate of MIF declines as the cancer progresses. Lymphocytes of the lymph nodes extirpated during operation show a similar tendency. 4. In comparing MIF activity of peripheral blood lymphocytes of gastric cancer patients before operation with that after operation, the activity in Stage II before operation is 41.7% as against 0% after operation, that in Stage III it is 29.0 % before operation and 10.5% after operation, showing amarked decrease, while that in Stage IV, in contrast, rises from 15.4% to 23.1%. Usually the MIF activity disappears when tumor removal is completely performed, but in the cases of progressive cancer the MIF activity seems to be sustained. 5. Looking at the cross reaction, in gastric cancer cases with positive MIF activity for autochthonous antigen they show 42.9% positivity against heterogenous antigen. Besides we have observed the cross reaction between gastric cancer and colon cancer as well as between colon cancer and gastric cancer. The foregoing findings indicate that in human cancer just as in animal cancer MIF activity first appears in the regional lymph nodes, which tends to decrease and disappears as the cancer advances. Since there can be observed a marked cross reaction between gastric cancer patients, MIF would serve as a useful criterion for the discovery of gastric cancer and for the determination of prognosis.