Two arginyl tRNA's from yeast (Arg tRNA I and Arg tRNA II) are separable by chromatography. Arg tRNA I which binds to the Escherichia coli ribosomes in response to the triplets CpGpU, CpGpC and CpGpA, transfers arginine only into the C-terminal position of the α -chain of rabbit hemoglobin (tryptic peptide α T17, position 141). Arg tRNA II, which binds to E. coli ribosomes in response to the triplets ApGpA and ApGpG, transfers arginine into position 31 of the α -chain (corresponding to peptide α T4). Codon assignments to positions 31 and 141 of the α -chain, based on the utilization of arginine from either of the two tRNA fractions, are consistent with known amino acid replacements at these positions, and could have been produced by a change of a single base. No labeled arginine was transferred into position 92 (corresponding to peptide α T11) nor into the two β -chain arginine positions. The lack of labeling of position 92 may be due to its specification by the triplet CpGpG, since neither of the tRNA fractions was bound to ribosomes in the presence of CpGpG. This codon assignment is consistent with the observed replacement of arginine at position 92 by leucine and glutamine in mutant human hemoglobins. The lower level of β -chains synthesis precluded a determination of the transfer specificity of β -chain arginine residues. Nevertheless, only the triplet ApGpPu at positions 31 and 41 of the β -chain can be interrelated with observed amino acid replacements, by a change of a single base.