Aim: Investigation of possible effects of static magnetic field (SMF) on adenosine deaminase (ADA) activity in cancerous and noncancerous human gastric and colon tissues to obtain information about possible action mechanism of SMF. Materials and Methods: Cancerous and noncancerous human gastric and colon tissues removed from patients by surgical operations were used in the studies. SMF was created by using two static magnets. Before and after treatment with SMF, ADA activities in the tissue samples were measured. Results: The ADA activity was found to be lowered in gastric tissues treated with the SMF. However, no change was observed in the ADA activity of colon tissues. Conclusions: Our results suggest that SMF inhibits the ADA enzyme in gastric tissues significantly. It is supposed that, in addition to other proposed mechanisms, accumulated adenosine due to the inhibition of the ADA enzyme might also play a part in the anticancer activity of SMF.