The newer methods of molecular virology, including molecular hybridization and the "simultaneous detection test," were used to examine human brain tumors for evidence of RNA tumor viruses. It was found that they contained 70S RNA and RNA-directed DNA polymerase, both encapsulated in a particle possessing a density of 1.17 g/ml. These particles therefore satisfy the three diagnostic features that characterize the animal RNA tumor viruses. Of 26 of the most malignant (glioblastoma and medullo blastoma) brain tumors examined, 24 (92%) contained these virus-like entities. The possible usefulness of these particles as aids in diagnosis and monitoring therapy is briefly discussed.