Determination of proliferative activity in tumours may be valuable in diagnosis and prognosis. In this study, commonly used proliferation markers were investigated and compared in 12 cases of human glioblastoma. Paraffin sections were incubated with four commercial Ki67-equivalent antibodies, anti-PCNA, and anti-bcl-2. S-phase fraction and mitotic activity were determined as well. The different Ki67 antibodies gave satisfactory immunostainings, though they provided a wide range of proliferation indices (PI) intra- and intertumorally. Correlations between the Ki67 antibodies and the other proliferation markers were, broadly speaking, poor. PCNA immunostaining was hampered by disturbing background staining. Few bcl-2-immunoreactive cells were observed, mainly gemistocytes. Flow cytometric analyses provided reliable S-phase fraction values, and two aneuploid tumours were detected. The mitotic activity was generally high. Thus, mitotic counting remains a convenient method for assessing proliferative activity in astrocytic tumours. Ki67 antibodies are important alternatives, for instance in stereotactic brain biopsies. Under all circumstances, proliferation markers should be used in combination with established histopathological criteria for malignancy in these tumours.