In an attempt to identify lymphocyte subsets possibly involved in the response to malignant cells, we have studied the lymphocyte surface phenotype by using a panel of monoclonal antibodies on both peripheral blood lymphocytes (PBL) and histologically proven metastatic and nonmetastatic (i.e., "hyperplastic") axillary lymph node lymphocytes (LNL) from eight breast cancer patients. Furthermore, we carried out a functional study by evaluating the response to polyclonal mitogens of the PBL and of the LNL of the same patients. A group of 30 healthy subjects, age and sex matched, were selected as controls for PBL. Six of them, who underwent surgery for nonneoplastic conditions, were selected as controls for LNL. The responsiveness of breast cancer patients' PBL to polyclonal mitogens phytohemagglutinin (PHA) and concanavalin A (Con A) was significantly lower as compared with the control response. The responsiveness of breast cancer patients' metastatic LNL was not different from control LNL for PHA, and it was lower than control LNL for Con A, while the responsiveness of the same metastatic LNL was higher than that of nonmetastatic (i.e., hyperplastic) LNL of patients. Furthermore, the response of hyperplastic LNL was always lower than that of control LNL. The responsiveness of patients' PBL was always lower than that of metastatic LNL, while the responsiveness of patients' PBL vs. hyperplastic LNL was at variance. Regarding the surface phenotype of PBL, there was no difference between those of breast cancer patients and controls concerning the T-cells subsets, while the Leu 7, CD 21 and DR antigens were significantly higher among the breast cancer patients. No significant differences were found between patient metastatic and hyperplastic LNL or between control LNL and patient metastatic or hyperplastic LNL, respectively; only the CD 4 antigen was higher in metastatic than in hyperplastic LNL. A comparison of this surface phenotype between PBL and either metastatic or hyperplastic LNL of breast cancer patients showed values almost constantly significantly higher for PBL vs. either metastatic or hyperplastic LNL, respectively. The results of our study suggest that there is no change in the local-regional immunocompetent cell subsets that may be related to metastasis of breast cancer to regional nodes and to the progression of disease and that circulating T cells in breast cancer include cells expressing activation markers but not showing significant changes in the proportion of entire subpopulations.