We have set out to establish a nomenclature for ribosomal proteins which transcends, as far as is possible, the species from which the ribosomes were isolated. Initally, we identified individual proteins of rat liver by two-dimensional polyacrylamide gel electrophoresis and comparison with the classification system established by others (Sherton, C.C., and Wool, I.G. (1972) J. Biol. Chem. 247, 4460-4467). This was possible with minor modifications. We carried out a detailed comparison of the rat liver proteins with those of chick liver; parallel separate and mixed preparations of ribosomal subunits from the two species were made. Spots on the two-dimensional gels were carefully scrutinized for possible species differences. Proteins which gave congruent spots for the separate and the mixed preparations were tentatively assumed to be identical between the two species. For these proteins, a species-independent nomenclature was adopted. Contrary to earlier reports, a number of proteins were found which were not identical between the two species. These were assigned a species-specific nomenclature. In all, 7 proteins in the small subunit and 17 from the large subunit of the rat liver ribosome were found to be significantly different in the chick. The evolutionary implications of this are discussed.