The sedimentation rates of ribosomal RNA from Escherichia coli, Bacillus subtilis, etiolated pea stem, white potato tuber, rabbit liver, rabbit reticulocytes, rat liver and rat fat-pads were compared by mixing any two of the above and analyzing them independently ( 32P and absorbancy at 260 mμ) on sucrose gradients. By this criterion rabbit liver, rabbit reticulocytes, rat liver and rat fat-pad ribosomal RNA's were indistinguishable. Pea stem and potato tuber as well as E. coli and B. subtilis ribosomal RNA's were also indistinguishable; in other words, within each small group of organisms studied, there was no detectable difference in sedimentation rate of the ribosomal RNA's. Differences were observed, however, in comparative studies of the major kingdoms. The sedimentation coefficients determined for plant tissue and animal tissue RNA were respectively, 16.5 and 24.7 s and 17.6 and 31.7 s using E. coli RNA (16.5 and 23.5 s) as a marker. The molecular weights for the above RNA's were also calculated, assuming that all ribosomal RNA's have similar conformation in solution. The data were calculated from the linear relationship M = KT α . The constant α was calculated from the peak tube numbers and molecular weights of E. coli RNA (5.6 × 10 5 and 1.1 × 10 6); K was calculated from the data for small ribosomal RNA of E. coli. The calculated molecular weights of the ribosomal RNA's were 6.6 × 10 5 and 2.1 × 10 6 daltons for mammalian tissue and 5.6 × 10 5 and 1.3 × 10 6 daltons for plant tissue. The relative amounts of the two ribosomal RNA molecules were determined by measuring the area under the profiles of the two molecules. The relative amounts (large/small) of ribosomal RNA (1.9 to 2.1:1 for E. coli, 2.1 to 2.3:1 for pea stems and 3.0 to 3.3:1 for rabbit reticulocytes or liver) were similar to the ratios of molecular weights for each of the organisms. This evidence further supports the values calculated for the molecular weights, since the ribosomal subunits apparently contain one molecule of RNA and are present in the cell in equimolar amounts; this requires that the two ribosomal RNA molecules also be present in equimolar concentrations.