The assessment of cell proliferation in neoplastic cell populations has been shown to be of prognostic value. The monoclonal antibody Ki67 is useful in establishing the growth fraction of neoplasms by quantitatively identifying Ki67 positive cells using immunohistochemistry. Its staining and prognostic value are comparable to the tritiated thymidine-labelling index. Ki67 positive cells give a more accurate indication of proliferating cells compared to that of PCNA positive cells, as PCNA is detectable in almost all quiescent cells adjacent to some tumours. The correlation between low and high Ki67 index and low or high-grade histology of neoplasms is strong. The monoclonal antibody Ki67 is effective on routinely-fixed, paraffin-embedded tissues after using the high temperature antigen unmasking technique, on frozen sections and cytological cell smears.