Red cell-mediated microinjection has been used to study tRNA turnover in SV3T3 mouse cells and TC7 cells, an African green monkey kidney line. The turnover of endogenous tRNA, measured by labeling with 3H-methionine, was first-order with half-lives of approximately one day in SV3T3 and two days in TC7 cells. 32PtRNA isolated from E. coli or TC7 cells turned over at the same rate as endogenous tRNA when injected into either SV3T3 or TC7 cells. This demonstrates that cellular processes, not properties inherent to tRNAs, are responsible for the difference in tRNA turnover observed between SV3T3 and TC7 cells. These results further indicate that the mechanism of tRNA turnover in mammaliam cells does not distinguish prokaryotic from eukaryotic tRNAs. In contrast to unmodified tRNA, glyoxalated tRNA was rapidly degraded upon injection. Thus altered tRNA's, like altered proteins, are turned over more rapidly in animal cells.