In 329 breast tissues (120 benign lesions and 209 invasive breast carcinomas) the growth fraction was determined using the monoclonal antibody Ki-67. The percentage of Ki-67 positive cells or the Ki-67 growth fraction was determined on histologic and cytologic specimens after immunoperoxidase staining. In benign breast lesions, the Ki-67 growth fraction never exceeded 11%, whereas in breast cancer 14.4 +/- 9.9% of cells were Ki-67 positive ranging from 1 to 48%. Breast carcinomas were classified according to the histologic grading system of Bloom and Richardson (G1-G3). Poorly differentiated carcinomas. (G3) had significantly higher Ki-67 growth fractions than well differentiated tumors (G1). Ki-67 was also correlated with axillary lymph node status. Growth fractions of N+ carcinomas were significantly higher than those of N0 tumors (16.4 +/- 10.4% versus 12.4 +/- 9.4%; p = 0.006). Thus, this new method yields similar results to those obtained by other researchers through the use of flow cytometry and thymidine labelling. As determination of Ki-67 growth fractions is an easy and rapid technique, it is ideal for routine clinical use. Ki-67 may be useful in prognosis and in the selection of patients for various treatment modalities such as adjuvant therapy. A wide-spread use of Ki-67 would be helpful to gain further experience.