The phenotype and activation status of lymphocytes from the peripheral blood and axillary lymph nodes of 40 patients with breast cancer were analysed using flow cytometry and compared with lymphocytes from the blood and lymph nodes of 7 control subjects. There was little difference in the overall proportions of T and B lymphocytes but there was a much larger population of B cells bearing surface IgG and a greater number of CD4+ helper T cells, particularly in the regional nodes, in the breast cancer patients. Many more T cells in the cancer patients were found to be carrying the HLA DR and Tac antigens. The axillary lymph nodes were the major site of B cells and CD4+ T cells whilst the primary tumour was the source of the CD8+ suppressor/cytotoxic T cells. Any immune response appeared to be largely loco-regional and may therefore destroyed by conventional surgery or radiotherapy.