1. Altered column-chromatographic profiles of a number of hepatoma and other tumor aminoacyl-tRNAs have been observed, most frequently for tRNAPhe, tRNAAsp, tRNAAsn, tRNATyr, tRNAHis, and tRNASer. 2. The chemical basis for the chromatographic alterations is still obscure, except that a tRNAPhe in Morris hepatoma 7777, which is not present in liver, was reported to lack the hypermodified peroxy-Y base. 3. Base composition analysis by the tritium derivative method shows hepatoma tRNA to exhibit a number of relatively small, but statistically significant differences in the amounts of various constituents when compared with liver tRNA. No overall hypermethylation of the tRNA in hepatomas and other tumors has been observed. Hepatoma tRNA rather is slightly undermethylated and undermodified, as compared to liver tRNA. 4. Differences in the base composition between Morris hepatoma and liver mitochondrial tRNA are greater than those between hepatoma and liver cytoplasmic tRNA. Hepatoma mitochondrial tRNA was also found to be undermethylated and undermodified. 5. The activity of the tRNA methyltransferases is increased in hepatomas and other tumors. 6. Urinary excretion levels of modified nucleosides and bases originating from tRNA catabolism are elevated in tumor-bearing hosts.