Proteins from Nicotiana tabacum cytoplasmic and chloroplast ribosomes and their subunits have been isolated under a variety of conditions and resolved by two-dimensional polyacrylamide gel electrophoresis. Average absolute mobility maps, constructed from the resultant electropherograms, were used to compare ribosomal proteins from cytoplasmic and chloroplast ribosomes. A novel technique for the estimation of molecular weights from two-dimensional electrophoretic mobilities is described. The cytoplasmic ribosome of N. tabacum possesses 73-80 distinct proteins with pI greater than 5, 26-30 associated with the 40 S subunit, and 47-50 with the 60 S subunit. The 60 S cytoplasmic ribosomal subunit has, in addition, at least three acidic polypeptides (pI less than 5). The chloroplast ribosome has 55-58 unique basic proteins with 22-23 occurring in the 30 S subunit and 33-35 in the 50 S subunit. A few additional acidic polypeptides are associated with the 30 S and 50 S subunits (2-3 and 1, respectively). There is little similarity between the electrophoretic patterns or molecular weight frequency distributions of proteins of analogous cytoplasmic and chloroplast ribosomal subunits of N. tabacum. The electrophoretic patterns and molecular weight frequency distributions of the proteins of N. tabacum chloroplast ribosomal subunits are quite similar to those of the Escherichia coli ribosome and the chloroplast ribosome of the alga Chlamydomonas. N. tabacum cytoplasmic ribosomal protein electrophoretic patterns and molecular weights are very similar to proteins from the Chlamydomonas cytoplasmic ribosome. These data substantiate: 1) the close affiliation between higher plant chloroplast and prokaryotic ribosomes and 2) a general similarity between angiosperm cytoplasmic ribosomal proteins and other classes of eukaryotic cytoplasmic ribosomal proteins.