Specific aminoacyl-tRNA's from different mammalian organs and species have been compared by methylated albumin column chromatography. The elution profile of such chromatographs were usually similar, both between tRNA's from different organs in the same animal, and across widely divergent mammalian species. Species differences were found in the elution profile of avian leucyl- and tyrosyl-tRNA's, but not among other tRNA's. In contrast to the similarities of methylated albumin-kieselguhr elution profiles of specific tRNA's from most mammalian organs, the elution profile of Ehrlich ascites tumor cells phenylalanyl-, seryl-, glycyl-and tyrosyl-tRNA differed from normal mouse organ tRNA. An extra peak of tyrosyl-tRNA has also been found in HeLa cells and adeno-7 virus transformed cells, but not in all tumor cells. The tyrosyl-tRNA of mammalian and chick fibroblasts in cell culture, and fibroblasts in the body exhibited a markedly different elution profile from the tyrosyl-tRNA of epithelial cells in culture and of normal organs. The tRNA differences discovered by in vitro charging have all been confirmed in vivo. Possible reasons for these variations are discussed. © 1968.