Mitochondrial ribosomal proteins (mrps) of the budding yeast, Saccharomyces cerevisiae, have been extensively characterized genetically and biochemically. However, the list of thegenes encoding individualmrps is still not complete and quite a fewof the mrps are only predicted from their similarity to bacterial ribosomal proteins. We have con-structed a yeast strain in which one of the small subunit proteins, termed Mrp4, was tagged with S-peptide and used for affinity purification of mitochondrial ribosome. .