Breast cancer tissue samples obtained from 147 Stage I and II patients were tested with the monoclonal antibody Ki-67 and avidin-biotin-peroxidase complex in frozen sections. The percentage of cells with nuclear staining ranged from 5% to 65%. The frequency of Ki-67 positivity was classified in five groups: 0% (45/147 = 31%); 5-9% (38/147 = 26%); 10-19% (15/147 = 10%); 20-39% (24/147 = 16%) and greater than or equal to 40% (25/147 = 17%). The mean value was 20%, median 18% with standard deviation of 14.5%. A significant positive correlation was observed between the presence of high Ki-67 nuclear staining rate with pathological tumor size (p = 0.003), histologic grading (p = 0.04), and axillary lymph node metastases (p = 0.009). An inverse significant correlation was found between Ki-67 and estrogen receptor expression (p less than 0.001). No correlation was observed with progesterone receptor expression or menopausal status. The overall picture is of an inverse relationship between high growth fraction determined with Ki-67 antibody and tumor differentiation parameters. These correlations confirm those already reported by thymidine labeling index and flow cytometry methods. The proliferative rate determined with Ki-67 antibody may provide information regarding cell kinetics of breast carcinoma, potentially useful in identifying patients with a different clinical course in order to improve the therapeutic approach, by a rapid, practical and easily performed immunohistochemical method.