We labeled the Ki-67 antigen in mammary carcinomas, using a MIB-1 monoclonal antibody, to evaluate the usefulness of MIB-1 Ki-67 Growth Fraction Indices (GFI; number of Ki-67 positive cells/total number of cells) of biopsy specimens in estimating the proliferative potential of the carcinomas. Formalin-fixed paraffin sections prepared from biopsy material, primary tumors, and axillary lymph nodes of ten invasive mammary carcinomas were chosen for immunohistochemical study. Bound antibody was detected using the avidin-biotin-complex peroxidase method. The GFI of the resected mammary carcinomas was similar to the estimated values based on the GFI of the biopsy specimens. The GFIs of the metastatic nodes in seven of the carcinomas were similar to those of the primary carcinomas, whereas two carcinomas yielded significantly different GFIs in the metastatic foci. These results suggest that the GFI of a mammary carcinoma biopsy specimen may reflect the proliferative ability of the whole carcinoma.