Breast cancer patients may express abnormal cellular immune responses affecting their immunological competence. The analysis of immunological parameters may be useful as indicators of T cell function. To determine the expression of lymphocyte activation proteins and cytokines in tumor and non-metastatic axillary lymph nodes, 30 breast cancer patients were monitored. CD3 polypeptides, PTKs (protein tyrosine kinases) and phosphorylated tyrosines were studied by Western Blot and cytokines mRNA expression was determined by RT-PCR (reverse transcription-polymerase chain reaction). This group of patient had shown high immunohistochemistry expression of IL-10 in tumors. Activation proteins were mainly expressed in involved lymph nodes comparing with their expression in tumors. The differences in expression of CD3 polypeptides and p56(lck) between both locations were significant. There was no statistical association between PTKs and IL-10 in the tumor but more than 50% of cases who express IL-10 lost p56(lck), p59(fyn). A direct association between IL-10 and CD3-polypeptides was observed, however 52.2% of patients who express IL-10 did not express 41 kDa CD3-zeta form. IL-10 mRNA was detected in more than 50% of tumors contrary to the prevalence of type 1 cytokines in regional nodes (40%). The lack of expression of lymphocyte activations proteins and the high expression of IL-10 suggest a downregulation on T cells function in the tumors. These results are useful in order to understand the local immune response that would be key in the control of the tumor progression.