Pure mitochondria, isolated from Euglena gracilis, will be used as a source of mitochondrial ribosomes, polyribosomes and messenger RNAs. These preparations will be used to determine (1) the genetic origin by DNA-RNA hybridization of those messenger RNAs translated on mitochondrial ribosomes and (2) the biochemical basis for specific requirement of mitochondrial ribosomes for homologous ribosomal proteins during the process of protein biosynthesis.