We studied the growth fraction of 55 resected colorectal adenocarcinomas by means of a three-step immunoperoxidase technique (avidin-biotin-peroxidase) using the monoclonal antibody Ki67 directed against a cell proliferation-associated nuclear antigen. The percentage of Ki67-positive cells was evaluated independently by two observers, and a Ki67 score was obtained for each case. No correlation was observed between Ki67 staining and patient's age and sex, tumor size and localization or grading and staging according to Dukes' method (modified by Astler-Coller and Turnbull). The growth fraction showed extreme heterogeneity in the cases examined, within each grade of differentiation.