The proteins of chloroplast and cytoplasmic ribosomes, isolated from Euglena gracilis, have been compared by electrophoresis on SDS-polyacrylamide gels. The proteins of the cytoplasmic ribosomes were more numerous and larger on the average than those of the chloroplast ribosomes. There were about 14 proteins detected in the small subunit of the chloroplast ribosome, ranging from 11,000 to 43,000 daltons and 16 proteins of 10,000 to 36,000 daltons from the large subunit. The banding patterns of the proteins of the subunits were quite distinct from each other. The subunits of the cytoplasmic ribosomes were obtained by dissociation of the monomer with EDTA, and in 100 mm and 500 mil KCl and the effects of these conditions of dissociation on the proteins of the subunits compared. Regardless of the means of dissociation, the small and large subunits each gave 20–21 proteins ranging from 10,000 to 49,000 daltons. However, a comparison of scans of the subunits indicated a selective and partial stripping of ribosomal proteins by high salt and by EDTA; i.e. different proteins were sensitive to the two treatments. Native subunits, presumed to occur free in the cytoplasm were also isolated. In addition to the ribosomal proteins found in small subunits obtained by dissociation, the native small subunit contained substantial amounts of high-molecular-weight proteins. Small, variable amounts of high-molecular-weight proteins are also associated with chloroplast ribosome subunits, but the quantities depend on the method of purification of the subunits. Because these components are virtually eliminated following two cycles of density gradient centrifugation, we infer that they are adventitious. These observations reflect on the relative merit among several reported methods of purification of chloroplast and cytoplasmic ribosomes.