The heterologous interaction of Escherichia coli ribosomal protein EL11 with yeast 26 S and mouse 28 S rRNA was studied by analysing the ability of this protein to form a specific complex with various synthetic rRNA fragments that span the structural equivalent of the EL11 binding site present in these eukaryotic rRNAs. The fragments were obtained by SP6 polymerase-directed in-vitro run-off transcription of parts of the yeast or mouse large rRNA gene cloned behind the SP6 promoter. EL11 was found to protect an oligonucleotide fragment of 63 nucleotides from both the yeast and mouse transcripts against digestion by RNase T1. In both cases, the position of this fragment in the L-rRNA sequence coincides almost exactly with that of the fragment previously found to be protected by EL11 in E. coli 23 S rRNA. Moreover, the protected yeast fragment was shown to be able to re-bind to EL11 by a nitrocellulose filter binding assay. A ribosomal protein preparation from Saccharomyces cerevisiae containing L15 (YL23) as well as the acidic proteins L44', L44 and L45 protects exactly the same oligonucleotide fragment as does EL11 in both the yeast and mouse transcripts. Evidence is provided that L15, which is known to be structurally and functionally equivalent to EL11, is the rRNA-binding protein in this preparation. Thus the structural equivalent of the EL11 binding site present in yeast 26 S rRNA constitutes the second example of functional conservation of a ribosomal protein-binding site on rRNA between prokaryotes and eukaryotes.