Two types of benzodiazepine receptors have been identified in mammalian tissues: a central type which is localized to neuronal elements in the brain, and a peripheral type which is present on glial cells and in tissues outside the central nervous system such as kidney. The authors report an increase in specific binding of peripheral benzodiazepine receptor ligands in certain human brain tumors using computer assisted quantitative image analysis of autoradiograms. Higher densities of binding sites to a 3H-labeled selective peripheral benzodiazepine ligand, PK11195 [1-(2-chlorophenyl-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide] were observed in human gliomas as the malignancy of these tumors increased. Specific binding was also present in some non-glial tumors but little binding was demonstrated in necrotic tissue or normal brain. In in vitro binding studies in rats, there was a significant increase in Bmax (1089.3 +/- 232.2 fmol/mg tissue) in C6 glial tumors and LK Walker 256 metastatic tumors (924.2 +/- 183.7) compared with normal brain (62.1 +/- 12.8 fmol/mg tissue). Binding affinities were, however, similar (Kd = 2.09, 2.17, and 2.04 nmol/l, respectively). These findings suggest that the number of peripheral benzodiazepine receptors are increased in brain tumors. These receptors could be utilized in positron emission tomography to image brain tumors.