The Ki-67 antigen is a cell cycle and tumor growth marker, which can be readily detected using immunocytochemistry methods. Ki-67 antigen is present only in the nuclei of cycling cells and is variably expressed as the cell goes through the various phases of the cycle: both the amount and the topographic distribution of Ki-67 antigen vary in a manner which is sufficiently specific to establish the precise cycle phase for each member of a cell population. From G1 through mitosis, the amount of Ki-67 antigen increases steadily: location of the antigen is nucleolar during G1 and both nucleolar and karyoplasmic during G2. The Ki-67 antigen is widely used to estimate the growth fraction of a tumor cell population; in many malignancies, the percentage of Ki-67-positive cells is correlated with parameters reflecting tumor aggressiveness or progression. These findings demonstrate the potential value of the Ki-67 antigen for the cytopathological or histopathological study of tumors, although neither its biochemical structure nor its function have been fully elucidated to date.