1. 1. Escherichia coli transfer ribonucleic acid was treated with HNO 2 at pH 4.3 and the relative rates of deamination of the bases, adenosine, cytidine and guanosine, were measured. Base ratios on partially deaminated ribonucleic acid were most accurately determined by alkaline hydrolysis and quantitative separation of the six nucleotides on Dowex-1 (formate form). 2. 2. Transfer ribonucleic acid samples, deaminated to different extents, were assayed for their ability to accept labeled amino acids, and as substrates for ribonucleic acid-adenylate (cytidylate) pyrophosphorylase (ATP (CTP): RNA nucleotidyl transferase). The phenylalanine acceptor was inactivated exponentially at a faster rate than either the lysine or valine acceptors, while the ability to reform the terminal pCpCpA sequence (after phosphodiesterase (orthophosphoric diester phosphohydrolase, EC 3.1.4.1) treatment) was reduced relatively slowly. Target sizes were calculated for these processes, assuming a random single-hit inactivation. 3. 3. Labeled aminoacyl transfer ribonucleic acid, when prepared from the partially deaminated transfer ribonucleic acid, transferred less of its bound amino acid into polypeptides than did normal aminoacyl transfer ribonucleic acid, when measured in a cell-free ribosomal system from Escherichia coli. Apparently the transfer function of a small percentage of the transfer ribonucleic acid population can be inactivated without affecting the acceptor ability. The various reactions exhibited by transfer ribonucleic acid depend upon different structural features, since they are inactivated at different rates by HNO 2.