The proliferative activity of 163 primary breast cancers was assessed by immunocytochemistry with the mouse monoclonal antibody Ki-67, which recognizes a nuclear antigen expressed in all phases of the cell cycle except for Go. The overall frequency distribution of Ki-67 staining was of exponential type, with percentage of positive staining cells ranging from 0.3 to 88.3%, with a median value of 10.1%. No relationship was observed between Ki-67 values and menopausal status of patients. A significant positive correlation was found between Ki-67 values and tumor grade, especially mitotic grade. Estrogen Receptors (ER) were assayed by immunocytochemistry with ER-ICA method and by dextran-coated charcoal method (DCC) in 129 and 141 tumors, respectively. A negative correlation was found between the ER content by both methods and Ki-67 score. Eighty-nine percent of cases with Ki-67 value less than 10.1% contained more than 10% ER-ICA-positive cells. Progesterone receptors (PgR) were assayed by immunocytochemistry with PgR-ICA method and by DCC in 62 and 141 tumors, respectively. A negative correlation was observed between the PgR content by both methods and Ki-67 score. No correlation was found between Ki-67 score and lymph node involvement by tumor. These findings suggests that Ki-67 score could be used as an independent prognostic marker, useful to distinguish different risk for recurrence within the two clinically heterogeneous groups of N- and N+ patients.