Using morphometric and immunohistological techniques, we investigated the distribution of T lymphocytes and subsets, B lymphocytes, macrophages and plasma cells in sections of human breast tissue. In normal lobules and ducts, leukocytes were found in greater density in the epithelium than in the stroma. The intra-epithelial cells consisted largely of suppressor/cytotoxic T lymphocytes with a smaller number of macrophages; inducer T cells were seen in only 1 out of 10 subjects and were present in very low numbers. In the stroma, there were roughly equal numbers of suppressor/cytotoxic cells, macrophages and plasma cells containing IgA or IgM. Small numbers of stromal inducer lymphocytes were seen in only 2 cases. Although the density of suppressor/cytotoxic cells in the epithelium was considerably greater than in the stroma, macrophages were present in roughly similar density in both locations. In carcinoma, the ratio of epithelial to stromal leukocyte density was reversed, due to a marked reduction in intra-epithelial cells and to an increase in those in the stroma. Between 10% and 30% of lymphocytes expressed activation markers in contrast to the normal breast in which virtually all were negative. Inducer lymphocytes were identified in 9/10 carcinomas. The possible relationship of inducer cells to breast carcinoma and pre-neoplasia is discussed and warrants further investigation. No alteration in stromal plasma cell numbers was observed.