Virchows Arch. A Path. Anat. and Histol. 365, 15--21 (1975) 9 by Springer-Verlag 1975 Immunofluorescence Detection of Casein in Human Mammary Dysplastic and Neoplastic Tissues G. Bussolati, A. Pith, and V. Aifani Istituto di Anatomia ed Istologia Patologica H, Universits di Torino, Torino, Italy P~eceived July 26, t974 Summary. To detect and localize casein in human mammary dysplastic and neoplastic tissues, an indirect immunofluorescence method has been devised. Anti-casein antibodies have been obtained from rabbits immunized with casein isolated from human milk. Cryostat sections post-fixed in alcohol and paraffin sections from routinely formalin-fixed tissues proved suitable. The immuno-fluorescence method revealed the presence of casein in epithelial cells in mammary dysplasia and in some, but not all, cases of carcinoma of the breast. Well differentiated (Grade I) carcinomas were shown to contain casein, mainly localized at the inner border of the epithelial cells; the milk protein was present also in cells metastatic in lymph nodes. In Paget's disease of the nipple, casein could be detected in neoplastic cells in the ducts and also in cells identifiable with the typical intra-epidermal Paget cells. This finding is evidence of a functional differentiation of Paget cells along lines proper to the mammary epithelium. Introduction The detection and cellular localization of milk proteins has been investigated in the rat (Turkington, 1969; Young and Nelstrop, 1970) and cow (Kihm, 1973) lactating mammary glands by means of immunofluorescence techniques: casein as well as other milk proteins could be detected. The former, presenting a marked antigenic resistance, could be localized also in formalin-fixed, paraffin embedded tissues. Tarkington (1970) furthermore applied immunofluorescence techniques to study the presence and localization of casein in experimental rat mammary tumonrs, to obtain information on the production of this specific endogenous protein, and therefore on the functional properties and homogeneity of tumonr cell lines. In the present work an immunofluorescence method for the localization of human casein has been devised and applied to the study of cases of mammary dysplasia and carcinoma. The detection of casein, a protein specific to the mammary epithelial cells, could in fact provide information on the secretory activity and the degree of differentiation of epithelial cells in human dysplastic and neoplastic lesions of the breast. In addition, since milk protein secretion is known to be under hormonal control, the presence of such proteins might possibly be an index of the status of hormonal balance and control in the tu- mours.