Individual ribosomal proteins S4, S9 and S13 were tested for their ability to interact with tRNA and synthetic polynucleotides. All three proteins bind to immobilized to Sepharose poly(A) and poly(U), while S4 and S13 form stoichiometric (1:1) complexes with tRNA in solution. We show that only the polynucleotide X S13 complexes are able to select their cognate tRNAs. In particular, the affinity of tRNAPhe to the binary poly(U) X S13 complex is about three orders of magnitude higher than that for poly(U) alone.