The density of omega 3 (peripheral type benzodiazepine) binding sites, a marker of reactive and tumoural cells, has been measured in different types of human brain tumours; omega 3 sites were quantified autoradiographically in sections from biopsy or autopsy specimens labelled with the specific radioligand 3H-PK 11195. Compared to normal brain parenchyma, up to 12-fold increase in omega 3 site densities were found in apparently viable areas of high grade astrocytoma and glioblastoma specimens, whereas more limited increases (2 to 3-fold) in this marker were observed in areas of necrosis. Low grade gliomas (astrocytomas) and meningiomas exhibited only moderate increases (2 to 3-fold) in this autoradiographic marker. Metastases of lung or kidney origin were characterized by greatly elevated (up to 20-fold) omega 3 site densities as compared to normal brain parenchyma. In every case, there was a good spatial correspondence between the histopathological limits of the tumour and the anatomical location of the increase in omega 3 site densities. These results suggest that omega 3 site densities in human brain tumours reflect their proliferative activity and point to a possible future usefulness of positron or gamma-ray emitting omega 3 site ligands for the clinical investigation and detection of human brain proliferative diseases.