The leucocyte inhibition factor (LIF) test, revealing the inhibition of leucocyte migration from capillary tubes, was studied with leucocytes from 47 patients with breast cancer, from 14 patients with benign breast tumors, and from 56 normal subjects. As antigens we used formalin-fixed tumor cells. The LIF assay was positive in 38/47 (80.8 %) breast cancer patients, in 10/14 (71.4 %) benign breast tumor patients, and in 13/56 (23.2 %) normal subjects. The negative LIF assays were higher with leucocytes from patients with metastases than with those from patients with no metastases. We found no correlation between the LIF-test response and the histopathologic pattern of the tumors, type of treatment (almost always surgical), and remission or relapse state of our patients at the time of testing.