The human breast is a highly specialized, complex tissue comprised of a heterogeneous population of cells with varying functions. Interactions between the different cell types, changes in their relative abundance, state of differentiation and function in response to stimuli, as well as the alterations that lead to the aberrant growth associated with malignancy are poorly understood. Two-dimensional gel electrophoresis is being used to compare the proteins found in different breast cells in order to identify the gene products that are common or specific to particular cell types so as to provide markers that will be useful in studies of normal breast cell differentiation and the dedifferentiation or blocked differentiation characteristic of cancer. Protein patterns have been obtained from cells prepared for electrophoresis immediately after isolation from human milk, from cells cultured for fewer than ten passages after isolation from healthy breast tissue removed during reduction mammoplasty, and from cells maintained in long-term tissue culture after isolation from the pleural effusions of patients with breast carcinomas. Differential expression of cytokeratins 8, 18, and 19, shown previously to be predominantly expressed by epithelial cells in the luminal layer of breast tissue, was observed among the cells analyzed. Other non-cytokeratin proteins were also found to be differentially expressed in subsets of both the normal and tumor cells. A composite human breast cell protein pattern was created which includes all the commonly and specifically expressed proteins found in this study. This pattern will be the basis for continuing studies of proteins in the human breast.