The antigen-induced leukocyte adherence inhibition (LAI) test is an assay of cellular immune reactivity which was expected to offer a promising adjunct to currently available diagnostic and monitoring procedures in cancer patients. We have applied this assay to (1) 83 inpatients clinically suspected of having breast cancer, (2) 50 outpatients before mammography, and (3) 37 healthy women. In order to account for the known heterogenicity of breast cancer, we performed LAI assays with extracts from 15 primary tumors of known and 14 primary tumors of initially unknown histology. Thus, the problem of tumor specificity in the LAI assay was tackled in a doubleblind fashion. The results obtained clearly show that breast-tissue-specific rather than tumor-specific responses were detected by LAI testing. The LAI assay failed to discriminate between high-risk patients with and without cancer and between extracts from breast tumors with and without carcinoma. The results are discussed with regard to the many claims for the detection of specific tumor immunity in the literature.