Mononuclear cell infiltration is frequently seen within human solid tumors. Effector cells within the tumor site usually fail to exhibit cytotoxic or natural killer activity when freshly isolated; however, they develop potent and sometimes specific cytotoxicity after expansion in ILL Thus, local tumor environment may influence lymphocyte function. In our study, we disaggregated human breast-cancer and lymphnode tissue to obtain lymphocyte-enriched cell fractions. Besides phenotypic analysis, functional characterization with regard to proliferation and cytokine production of tumorinfiltrating lymphocytes (TIL), peripheral-blood lymphocytes (PBL) and lymph-node lymphocytes (LNL) was the aim of our study. TIL showed an enrichment of CD8+ cells with a corresponding decrease in CD4+ cells in comparison with PBL and LNL. In response to PHA, TIL showed decreased 'H-thymidine uptake, but TIL were significantly stimulated by rhlL2. TIL produced low levels of ID, TNF and IFNy upon mitogen/ phorbol ester stimulation, while PBL produce high levels of TNF and IFNy but low levels of IL2. Under the same experimental conditions, LNL produce high levels of TNF and 1L2 but low levels of IFNy. Mitogen-mediated TNF secretion was increased after addition of autologous tumor cells in TIL and LNL, whereas IFNy secretion tended to be suppressed. Our results indicate different patterns of activities of TIL, LNL and PBL from breast-cancer patients.