VARIATIONS in isoaccepting tRNA populations occur in mammalian cells during development, neoplasia, and virus infection, and such changes may reflect the involvement of tRNA in regulatory processes1,2, although firm evidence to that effect is not available. Ortwerth and Liu3 have shown that in normal or neoplastic cells an isoaccepting species of lysine tRNA, tRNA4Lys, is present in dividing cells but absent from non-dividing cells. Ortwerth et al.4 determined some of the functional properties of tRNA4Lys from various tissues and obtained evidence that the species is a real isoacceptor of tRNALys rather than an artefact. Normal, proliferating cells that were examined had only relatively small amounts of tRNA4Lys; however, a larger proportion of the isoacceptor was found in neoplastic cells such as mouse leukaemia and Morris hepatoma cells. It is desirable, therefore, to establish whether the large amount of tRNA4Lys is peculiar to the neoplastic state or whether normal cells may also have large amounts of that isoacceptor under some growth conditions. Therefore, we studied isoaccepting lysyl-tRNA profiles from mouse cells in different states of proliferation: adult liver, embryo and growing and quiescent primary cultures of embryonic cells. We found that normal growing cells also have an appreciable amount of tRNA4Lys.