Various eucaryotic tRNAs were compared from the standpoint of their ability to misread poly-U in the tRNA-dependent cell-free system of wheat germs. The highest level of 14C-leucine incorporation into the product of translation is observed for tRNA from differentiated (lactating) mammary gland under conditions of the increased Mg2+ concentration. Possible reasons may be connected with different tRNALeu/tRNAPhe ratios in these tissues and with the appearance of specific isoacceptory tRNA4Leu in the total set of tRNA from the lactating gland.