The monoclonal antibody Ki 67 is able to detect a nuclear antigen expressed by proliferating cells during the cellular cycle (phase G1, G2, S and M). It is used as a marker of the proliferative activity of 60 cerebral gliomas including 41 biopsied under stereotaxic conditions. The immunocytochemical study performed with the peroxidase antiperoxidase technique on crushed and smeared fresh tumors, permit to define a nuclear marking index for each tumor. The index Ki 67 is correlated with the degree of tumor malignancy. The maximum values are found in astrocytomas IV (20%) and malignant ependymomas (18%), with a mean index at 11% in the 25 high-grade gliomas and 1.7% in low-grade gliomas. The minimum values are observed in pilocytic astrocytomas and low-grade oligodendrogliomas. Marked variations of the index Ki 67 are found in homogeneous tumor classes regrouping tumors of identical nature and grade: grade IV astrocytomas (5 to 20%), grade III astrocytomas (8 to 18%). These marking differences reflect the heterogeneous nature of the proliferative activity of morphologically similar gliomas. The nuclear marking index with antibody Ki 67 complements the findings of the standard histological examination of cerebral gliomas. This is an additional diagnostic mean to evaluate the proliferative potential and the prognosis of these tumors.