Methods of assessing tumor proliferation rates include mitosis counting, flow cytometry and thymidine labelling. While the former is inaccurate and poorly reproducible, the latter methods are time consuming and expensive to perform. Ki-67 is a monoclonal mouse antibody which has been shown to react with a nuclear antigen in proliferating cells. Frozen sections from 75 specimens of breast carcinoma were immunostained with this antibody using an immunoperoxidase technique. The percentage of tumor cells stained, the Ki-67 score, was then compared with a number of pathological and clinical variables in the patients concerned. A positive correlation was seen between the Ki-67 score and mitotic rate (r = 0.71); and a negative correlation was seen between Ki-67 score and estrogen receptor status (r = -0.4). Ki-67 immunostaining may represent a cheap and reproducible method of assessing proliferation rates of breast carcinomas which is applicable in routine laboratories. Further prospective studies are being undertaken to assess its contribution to prognosis.