The primary structure of homogeneous low molecular RNA preparation with the sedimentation coefficient 2.5S isolated from amilose isomerase (A1) of muscle (E.C. 18.104.22.168) was analysed. This RNA can be digested by venom phosphodiesterase as well as by pancreatic and T2 RNAases; hence we conclude that the polynucleotide chain of A1 RNA consists of 3.5-phosphodiester bonds common for all RNAs. The nucleotide composition of the RNA was studied by two-dimensional TLC followed by spectrophotometry. The results show that its chain is 31--32 nucleotides long. High content of unusual components (about 30%) and guanine (about 40%) are specific features of this RNA.