Chloroplasts and proplastids isolated respectively from autotrophic and dark-adapted cells of Euglena gracilis strain Z incorporated (14)C-l-leucine into protein. In each case the incorporation was inhibited by chloramphenicol (50% inhibition at about 5 mug/ml for chloroplasts and 30 mug/ml for proplastids), but not appreciably by cycloheximide at concentrations up to 200 mug/ml. Chloroplasts from autotrophic cells incorporated leucine into protein at rates of about 10 pg leucine per mg RNA in one minute, but isolated proplastids were only 5 to 10% as active. When dark-adapted cells were illuminated there was little increase in the activity of the chloroplast fraction during the first 12 hr. Between 12 and 24 hr, when there was a rapid increase in the rate of synthesis of chlorophyll, the capacity of the chloroplast fraction for protein synthesis increased markedly. Suppression of the formation of a chloroplast-localized system for protein synthesis by treating the cells with chloramphenicol and the lack of such an effect with cycloheximide suggests that certain of the proteins which form part of a functional chloroplast system for protein synthesis are themselves synthesized within the chloroplasts.