Lymphocytic infiltrates were isolated from normal, preneoplastic, and neoplastic mouse mammary tissues. The surface markers on the infiltrating lymphocytes were characterized by immunofluorescent staining and flow cytometry. Preneoplastic and neoplastic tissues contained 10- to 20-fold more in situ lymphocytes than did the normal pregnant gland. Most of these lympocytes were T-cells. Relative to the T-cells in normal gland, the T-cells in C4 preneoplastic hyperplastic alveolar nodules and their spontaneous tumors have shifted in favor of the killer-suppressor subpopulation. This shift of T-cell subpopulations was a localized phenomenon and was not seen in the lymph nodes of hyperplastic alveolar nodules and tumor bearing mice. C4 lesion infiltrating cells also contained a subpopulation of lymphocytes that expressed 5- to 6-fold more LFA-1 antigen (lymphocyte function associated antigen-1) than did normal lymph node cells. The infiltrating lymphocytes of mammary tumors from cloned cell lines, on the contrary, had the same staining profile as did the lymphocytes from normal gland. Since most studies with human breast cancer infiltrates have demonstrated increased killer/suppressor T-cells and the presence of activated lymphocytes (J. Hurlimann and P. Saraga, Int. J. Cancer, 35: 753-762, 1985; H.L. Whitwell, H.P.A. Hughes, M. Moore, and A. Ahmed, Br. J. Cancer, 49: 161-172, 1984; and J.A. Ledbetter, R.V. Rouse, H. Spedding Micklem, and L. Herzenberg, J. Exp. Med., 152: 280-295, 1980) the C4 hyperplastic alveolar nodules and spontaneous tumor system may be a more relevant model for studying breast cancer infiltrates.