The monoclonal antibodies BA 16 and BA 17 directed to different epitopes on human keratin 19 have been tested for their reaction with normal breast and with benign and malignant breast lesions and associated tissue. In Western blots of gel‐separated extracts of fibroadenomas, malignant tumours or normal mammary epithelial cells, the antibodies reacted with only one component of 40 kd molecular weight. Immunoperoxidase staining of sections of normal breast tissues showed all basal cells and a few luminal cells to be unstained by the antibodies. The distribution of the unstained (keratin 19−) luminal cells in the mammary tree is consistent with that of cells with the proliferative potential to give rise to the growth of terminal ductal lobular units (TDLU) seen at pregnancy. A total of 42 benign and 141 malignant lesions were stained with the antibodies, and a clear difference in staining pattern was seen between the benign and malignant tumours. All but 3 of the benign lesions showed a heterogeneous staining pattern with 5–50% unstained cells. In contrast, the cancer cells in 106/116 invasive primary tumours and in all 21 metastatic lesions examined showed a homogeneously positive reaction with antibodies BA16 and BA17: the malignant cells in 4 cases of Paget's disease also showed homogeneously positive staining with the antibody. In the malignant tumours, the observed homogeneity in expression of keratin 19 was confined to the malignant cells; tumour‐associated normal tissue and benign proliferative lesions contained keratin 19−cells. Seven pure in situ tumours were examined and 5 showed the homogeneous pattern of staining characteristic of invasive tumours while 2 contained a high number of keratin 19− cells. A general model is presented to explain the presence of keratin 19− cells in benign proliferation and the dominance of keratin 19− cells in invasive carcinoma.