The oxy-radical scavenger enzyme manganese superoxide dismutase (MnSOD) may act in the capacity of a tumour-suppressor gene. To address the issue of its role in tumour transformation and progression in vivo, we evaluated the content of this enzyme in 33 brain tumours of neuroepithelial origin with different degrees of differentiation (WHO grade II-IV) by means of Western blot and immunohistology. Our results show that immunoreactive MnSOD increases in a direct relationship with tumour grade and is therefore inversely correlated with differentiation. The increase in induced at a pretranscriptional level and is apparently specific to brain tumours of neuroepithelial origin. Approximately 30% of grade IV tumours display low levels of MnSOD content, and preoperative radiotherapy and brachytherapy result in low amounts of enzyme. Based upon these observations, we suggest that MnSOD cannot be considered a classical tumour-suppressor gene.