Queuosine (Q nucleoside) is a highly modified guanosine analog and is present only in the first position of the anticodons of tRNA(Tyr), tRNA(His), tRNA(Asn) and tRNA(Asp). The levels of Q in normal human brain and two different types of human brain tumors (meningiomas and astrocytomas) were determined by using an enzymatic assay method. tRNAs isolated from tumor tissues contained decreased amounts of Q when compared to tRNAs from normal (i.e. non-tumor) brain tissue. There was a relationship between the levels of Q nucleoside and the tumor malignancy in the sense that the deficiency was greater in the highly malignant astrocytomas compared to meningiomas which are usually benign tumors. Increased Q deficiency was also observed in higher grade tumors.