Nucleotide sequences in serine-specific transfer ribonucleic acids The results of the preceding papers on the isolation of several serine t-RNA fractions and their splitting with pancreatic and T 1-ribonucleases (EC 2.7.7.16 and EC 3.1.4.8 respectively) are discussed. By countercurrent distribution two serine t-RNA's (I and II) were largely separated from each other; they also differed in behaviour during methylated albumin-kieselgur column chromatography. The oligonucleotide patterns of serine t-RNA's I and II, on the other hand, were very similar. The nucleotide sequences found in the serine t-RNA's are summarized in Table I. Several overlapping sequences were found, with the odd nucleotides and the (Ap) nG-sequences as links. Some of the 11 odd nucleotides occur clustered in the RNA chains. Various possible “anticodons” appear in the RNA. There is very little similarity between the sequence model of serine t-RNA, proposed by Cantoni et al., and the nucleotide sequences of the serine t-RNA's determined here. Possible base pairing between the nucleotide sequences of the serine t-RNA's (Table I) are discussed.