The reactivities of leukocytes from gastric cancer and noncancer patients to gastric tumor and normal tissue extracts were tested by the leukocyte adherence inhibition (LAI) microtest, assessing cell-mediated immunoreactivity to adenocarcinoma of the stomach. The reactivities were expressed with the LAI index. All leukocyte preparations showed low reactivities, a LAI index of less than 20%, to normal tissue extracts and only the preparations of leukocytes from cancer patients displayed high reactivities, a LAI index of more than 20%, to tumor extracts. Assuming that a patient is sensitized to gastric tumor antigen if his leukocytes respond to at least one tumor extract with a LAI index of more than 20%, approximately half of the cancer patients tested in the present study are considered to have positive cell-mediated immunoreactivity to gastric tumor antigen. Thus, the LAI microtest appears to be a simple, rapid and specific method for demonstrating cell-mediated immunity to tumor.