A series of short oligo-DNA probes (8-10-mers) complementary to various regions of the plant ribosomal 5S ribonucleic acid (5S rRNA) have been synthesized. The results of their hybridization to free 5S rRNA and to ribosomes pointed to the availability of nucleotides in loop "C" for complexation. We found a correlation between hybridization of selected oligonucleotides and their inhibitory effect on enzymatic binding of Phe-tRNA and poly(Phe) synthesis on wheat germ 80S ribosomes. Evidence was obtained for involvement of 5S rRNA in the elongation of polypeptide chain during protein biosynthesis. 5S rRNA seems to play a critical role in protein biosynthesis, probably through causing conformational changes of loop C.