Abstract An improved method for the isolation of Euglena chloroplast ribosomes is described which presents a number of advantages over past procedures. First, ribosomes are prepared from whole cell extracts, thus bypassing the need to isolate intact chloroplasts and resulting in a 10-fold improvement in yield. Second, the inclusion of 40 m m Mg 2+ in the preparation buffers, while stabilizing the chloroplast ribosomes, precipitates and, thereby, virtually eliminates the cytoplasmic 89 S ribosomes. Third, greater than 95% of the chloroplast ribosomes sediment at 68 S rather than as the damaged 53 S particle frequently generated in other preparation procedures. Fourth, even with a high-salt wash to remove endogenous factors, the chloroplast ribosomes still sediment at 68 S and are just as active in in vitro protein synthesis as are E. coli ribosomes. These ribosomes have been tested for activity with elongation factors from prokaryotes, eukaryotes, and the chloroplast itself, and the results have been compared to those obtained with E. coli and wheat germ ribosomes. The data may be summarized as follows: (a) Chloroplast ribosomes use E. coli EF- Tu Ts and EF-G with the same efficiency as do E. coli ribosomes in protein synthesis, (b) E. coli and chloroplast ribosomes can use Euglena chloroplast EF-G to catalyze translocation, but wheat germ ribosomes cannot, (c) Wheat germ EF-1 H and EF-2 are highly active in polymerization with wheat germ ribosomes, but ribosomes from neither E. coli nor the chloroplast are able to recognize these factors, (d) All three types of ribosomes accept Phe-tRNA from E. coli EF-Tu although to differing degrees. However, neither chloroplast nor E. coli ribosomes recognize wheat germ EF-1 H for the binding of Phe-tRNA.