Tumor antigen-induced inhibition of leukocyte adherence was adapted and modified for use in glass test tubes for the study of cell-mediated antitumor immunity to human adenocarcinoma of the breast. Peripheral blood leukocytes from 40 to 47 patients with proven breast cancer responded to an antigenic extract of breast cancer with significant leukocyte adherence inhibition, whereas only 2 of 32 controls showed a response. Further, 7 patients with histologically proven benign breast disease did not react to the breast adenocarcinoma extract, indicating that only breast cancer patients have leukocytes sensitized to the breast cancer antigen. The cell-mediated antitumor response of the breast cancer patient was dependent on the stage of the cancer, and patients with disseminated cancer had decreased responsiveness. In fact, 4 of 7 breast cancer patients who had no response in the assay had disseminated breast cancer. Also, surgery and irradiation depressed leukocyte adherence inhibition responsiveness. Chromatographic fractionation on Sepharose 4B of the breast cancer extract showed that the antigenic component was greater than 10(6) daltons. The responsive cell in the assay interacts directly with the tumor antigen, and as a result subsequent adherence to glass is inhibited. The assay described is a comparatively simple and sensitive technique for demonstrating cell-mediated antitumor immunity and appears to be immunologically specific.