Two of the main biochemical features characteristic for oncogenic RNA (oncorna) viruses have been detected in human glioblastoma. In all tumors tested so far a RNA instructed DNA polymerase (“reverse transcriptase”) activity was present which exhibited the criteria specific for oncorna viruses. It was stimulable by the synthetic polynucleotide poly-rA: oligo-dT, it was almost insensitive to actinomycin D but very sensitive to ethidiumbromide. Using the simultaneous detection test, a high molecular weight RNA species was found in the subcellular fraction with the highest reverse transcriptase activity.