The covalent structure of rat ribosomal protein L5, which associates with 5 S rRNA in the organelle, was deduced from the sequence of nucleotides in a recombinant cDNA (pL5-6-4) and confirmed from the sequences of amino acids in portions of the protein. Ribosomal protein L5, encoded by pL5-6-4, contains 296 amino acids and has a molecular weight of 34,298. However, a second recombinant cDNA, pL5-8-5, encodes a protein with an additional methionyl residue at position 236 and may be the product of a second active L5 gene. Rat L5 is homologous to yeast YL3 and to Halobacterium cutirubrum HL13, proteins that also bind to 5 S rRNA. No significant structural similarity, however, was found between rat L5 and other 5 S rRNA-binding proteins; not with a second H. cutirubrum protein HL19, nor the Escherichia coli ribosomal proteins, L5, L18, or L25, nor the Xenopus laevis transcription factor IIIA. H. cutirubrum HL19, however, has structural identity with E. coli L5 and seems to be related to yeast YL3 and, hence, may be an evolutionary link between the prokaryotic and eukaryotic 5 S rRNA-binding proteins. A group of ribosomal proteins not known to be associated with 5 S rRNA are also related to rat L5. They include: rat L39, Euglina gracilis chloroplast S7, Saccharomyces cerevisiae L31 and L46, Homo sapiens L32 and, perhaps, several others as well. There is an especially close interrelationship between rat L5, rat L39, yeast L46, human L32, and mouse L32. These results, and others, suggest that ribosomal proteins form an extended family and that L5 may contain in its structure traces of this affinity.