Paraffin-embedded tissue slides from 89 infiltrating ductal breast carcinoma, 10 fibrocystic disease and 10 fibroadenoma were assessed immunohistochemically using monoclonal antibodies against human p65 antigen and polyclonal antibodies against p65-like protein present in fetal bovine serum. We did not find any evident differences in p65 detection by polyclonal and monoclonal antibodies, however, monoclonal antibody seems to be more specific. This factor is not induced by cellular proliferation associated with nonneoplastic diseases what was confirmed by immunohistochemical analysis of expression of p65 protein and well know markers of proliferation (proliferating cell nuclear antigen--PCNA and Ki67). It was established that there is no correlation between p65 and PCNA or Ki67 expression. High proliferating indexes (PI) for PCNA (PI-PCNA) or Ki67 (PI-Ki67) may help in selection of tumors with high proliferating activity independently from histological grade of malignancy established by routine methods. The estimation of p65 protein may be useful in the selection of precancerous changes and more differentiated ductal cancer of the breast what raises the possibility that p65 antigen may be helpful in the screening examination of women with high risk for cancer development.