Abstract 1. 1. The effects of four inhibitors of protein synthesis (puromycin, cycloheximide, chloramphenicol and tetracycline) have been examined in Acetabularia. Morphogenesis, protein and RNA synthesis have been studied in nucleate and anucleate fragments; other experiments have been made on chloroplasts isolated from anucleate fragments. 2. 2. Tetracycline (as well as chloramphenicol) strongly inhibits protein synthesis in isolated chloroplasts, while cycloheximide has no effect. 3. 3. All four antibiotics strongly inhibit the uptake of amino acids and their incorporation into the proteins of the chloroplastic and non-chloroplastic fractions of living algae. The inhibition is of the same order of magnitude in nucleate and anucleate fragments. The antibiotics, with the exception of cycloheximide, also inhibit the synthesis of chloroplastic and cytoplasmic RNA. 4. 4. Puromycin and cycloheximide inhibit regeneration, irreversibly in anucleate fragments, reversibly in nucleate ones. Tetracycline and chloramphenicol allow the formation of caps in anucleate fragments, but slow down their growth. 5. 5. The significance of these results is discussed.