Using immunohistochemistry, the epithelial expression of HLA Class I and II antigens (and beta 2 microglobulin) was compared in benign and malignant breast, and the stromal lympho-histiocytic infiltrate of these tissues quantified. The findings were compared with certain characteristics of the carcinomas. In contrast to other studies, malignancy was found to be associated with a far greater infiltrate of both lymphocytes and macrophages. In carcinomas, lymphocyte but not macrophage numbers showed a positive correlation with epithelial HLA ABC and HLA DR expression. This was particularly striking for T lymphocytes, and especially for the T4 subset. The histological grade of carcinoma is closely related to the degree of HLA ABC and beta 2 microglobulin expression, but no such relationship was found for HLA DR. There were greater numbers of mononuclear cells in the poorer differentiated tumours compared with other grades. Those carcinomas with evidence of lymph node metastasis contained greater numbers of macrophages identified by the antibody Y1/82A, but none of the other parameters studied was found to be associated with local lymph node status. The use of a large panel of monoclonal antibodies against various cellular subtypes has allowed a more detailed analysis of the interaction between breast epithelium and the host response.