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Synopsis Protein synthesis by cytoplasmic ribosomes from plants is not inhibited by any of these antibiotics. On the other hand, cytoplasmic ribosomes are inhibited by cycloheximide, which does not affect the activity of chloroplast ribosomes. It must be emphasized that all these findings relate only to the activity of isolated sub-cellular systems; it cannot be assumed from these data alone that it is valid to use these compounds on intact cells with the expectation of inhibiting protein synthesis in one compartment but not in the other. The protein complements of chloroplast and bacterial ribosomes are different and thus the above similarity between them must reside in their antibiotic binding sites, and not in the properties of the proteins as revealed by gel electrophoresis or immunological tests.