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Test for cell-mediated antitumor immunity and related serum blocking factors based on the finding that leukocytes from cancer patients, but not from controls, when mixed in vitro with antigenic extracts of tumors of the same histological type, undergo a diminution in their normal adherence to glass surfaces. Sera from tumor-bearing patients block the LAI reaction of their own leukocytes or those of other patients with the same type of tumor.