1. Calf liver and spleen transfer RNA's were each aminoacylated using either calf liver or spleen aminoacyl RNA ligases (EC class 6.1.1) and 14C-labeled or 3H-labeled valine, glycine, serine, phenylalanine, lysine and histidine. 2. Mixtures of liver RNA aminoacylated with 14C-labeled amino acid and spleen RNA aminoacylated with the same 3H-labeled amino acid were fractionated on methylated bovine serum albumin columns. Identical 14C and 3H radioactivity profiles were obtained with the amino acids valine, glycine, serine, phenylalanine or lysine. 3. In contrast, using [ 14C]- and [ 3H]histidine, a different histidinyl RNA was detected in liver transfer RNA aminoacylated with liver or spleen enzymes.