Immunostaining with monoclonal antibody Ki-67 (MAb Ki-67) has been employed to determine the growth fractions in a series of 139 primary adenocarcinomas of the large bowel. A wide range (18.9-71.4%; mean 39.4%; median 37.2%) in the percentage of Ki-67 reacting cells (Ki-67 index) was observed. The Ki-67 index was found to be unrelated to tumour stage, lymph node involvement, and presence of synchronous distant metastases. Mucinous carcinomas showed higher levels of Ki-67 reactivity than non-mucinous adenocarcinomas (P = 0.0003). Among non-mucinous adenocarcinomas a significant inverse correlation was demonstrated between the percentage of Ki-67 stained cells and the degree of differentiation (P = 0.002), and preservation of nuclear polarity (P less than 0.001). Moreover, tumours of patients younger than 45 years were generally characterized by high numbers of proliferating cells. There was no correlation between Ki-67 index and the other clinical and pathological variables examined. In most cases small differences in Ki-67 reactivity were observed in different samples from the same tumour. These results demonstrate that immunohistochemical assay with MAb Ki-67 represents a simple and reliable method for the assessment of proliferative activity in colorectal adenocarcinomas and suggest that Ki-67 labeling may provide information of clinical relevance in the management of patients with large bowel cancer.