By the monoclonal antibody PC10, the Authors have analyzed the proliferating cell nuclear antigen (PCNA) immunoreactivity on fixed and unfixed specimens of breast ductal (42 cases) and lobular carcinomas (15 cases) as well as in normal breast tissue adjacent to carcinomas; for all cases the post-surgical staging as well as immunohistochemical data concerning ER and PgR receptors were available. In addition, the immunohistochemical pattern of Ki-67 has been also recorded. The study documented a constant expression of PCNA in all breast carcinomas with an heterogeneous pattern whereas the immunoreactivity for Ki-67 appeared less represented and focal, so that a linear relationship between PCNA and Ki-67 immunostaining was absent. Moreover, the chi-square test failed to show any correlation between PCNA and immunocytochemical demonstration of ER and PgR receptors, dimensions of tumours, histological grade and lymph node involvement. The PCNA expression in adjacent morphologically normal breast tissue may suggest an accumulation of cyclin without a necessary induction of DNA synthesis.