A total of 170 axillary lymph nodes were obtained from fresh mastectomy specimens from 81 women with breast cancer. Lymph node cells were tested in vitro for T and B cells by the rosette technique and immunofluorescence microscopy and for functional capacity by response to the mitogens phytohemagglutinin (PHA) and concanavalin A. T cells showed a wide range of relative values: 32-80 percent, with a mean of 63.5 percent. B cells defined by the presence of surface immunoglobulins ranged from 14 to 61 percent (mean, 35.8 percent); those defined by the presence of C3 receptors, from 8 to 54 percent (mean, 24.9 percent); and those defined by the presence of IgG-specific (Fc) receptors, from 10 to 45 percent (mean, 27.5 percent). Cells with the C3 and Fc receptors constituted approximately two-thirds of the cells not binding spontaneously to sheep red blood cells (non-SRBC-R), whereas virtually all non-SRBC-R stained for surface immunoglobulins. The proportion of T and B cells and the response to mitogens varied widely among nodes and among patients. Differences were significant between values observed in young and old patients, nodes with and those without metastatic disease, and lymph nodes with different morphology. Lymph nodes from patients over 60 years old showed a higher proportion of B cells and a lower proportion of T cells than did those from patients 45 years of age or younger. Lymph nodes with disease metastic to them also showed a higher percent of B cells and a lower percent of T cells than the nodes that did not have metastatic disease. Lymph nodes with lymphocyte predominance showed a relatively high proportion of T lymphocytes, a high PHA response, and a low content of B lymphocytes. By contrast, lymph nodes with germinal-center predominance showed a relatively low content of T cells, a low PHA response, and a relatively high proportion of B lymphocytes.