1) Efficient separation of the proteins from rat liver ribosomes can by achieved by two-dimensional polyacrylamide gel electrophoresis. Complete separation of all components, however, is not possible with one system only. Comparison of the results obtained with different systems suggests further heterogeneity of S15, L22, L28, L33 and L35 and enables identification of S15a, S15b, L22a, L22b, L28a, L28b, L33, L33a, L35a and L36b. 2) Ribosomal proteins were substituted with iodoacetamide prior to electrophoresis or handled in all steps of the procedure in the presence of reducing agents. These procedures prevent the formation of oxidation products described erroneously as ribosomal proteins S5, S6, L15, L17 and L32 in earlier papers. 3) Estimation of the molecular weights was performed by two-dimensional separation of the small and large subunit proteins using sodium dodecyl sulphate in the second dimension. The positions of the 70 basic proteins in the 2-D patterns were identified. 4) The small and large subunit proteins have molecular weights in the range of 8000 to 35,000 and 11,000 to 55,500 Dalton, respectively. The number average molecular weights for the small and large subunit proteins are 22,500 and 26,500 Dalton, respectively. The sum of the molecular weights is 0.67 x 10(6) Dalton for the proteins of the small subunit and 1.05 x 10(6) Dalton for the proteins of the large subunit.