Natural cytotoxicity against CLA-4 and D6 target cells was determined in the peripheral blood of healthy donors and women with mammary carcinoma (localized to the breast and axilla), and in human normal and regional tumour lymph nodes. The NK cell activity in the blood of women with clinically localized mammary carcinoma showed a wide range of reactivities but was similar to the pattern detected in healthy controls. The NK cell activity in human normal lymph nodes also showed a wide range and was the same irrespective of the anatomical site of origin of the lymph node. Lymph nodes draining a variety of solid tumours had a similar spectrum of NK cell activity, there being no obvious correlation between the level of NK cell activity and such parameters as size and histological type of tumour, hyperplasia or metastatic spread in the regional node. Trypsin treatment of lymphocyte suspensions from the various lymphoid compartments suggested that the cell responsible for natural cytotoxicity in blood was probably different from the NK cell in tonsil and lymph nodes.