The leukocyte adherence inhibition (LAI) method was studied with respect to its specificity in detecting responses to extracts of tumor tissues or normal lymphocytes. Responses of cells from normal persons were within 10% of each other whether incubation was carried out with culture medium alone or with any of the extracts. The same was true of cells from 78 cancer patients unless the cells were incubated with extracts of the same histologic type as their own. In the latter case, statistically significant responses occurred in 95% of the 110 analyses done. Negative responses were given by cells from 14 patients tentatively diagnosed as having breast carcinoma but whose lesions later proved benign. There was one positive response inconsistent with the diagnosis. Of 29 normal individuals known to have been exposed to tumors or tumor extracts, 11 responded positively and specifically to the relevant tumor extract. Cells from 12 of 30 multiparous female breast-cancer patients responded to extracts of pooled normal lymphocytes. The results establish that the LAI analysis is an extremely specific means of detecting systemic responses to malignant diseases. In addition, analyses have proven positive in 95% of the cases studied.