Improvements are described in the preparation and in vitro conditions of an intact pea (Pisum sativum Progress No. 9) chloroplast system which provides high efficiency for translation of endogenous messenger RNA, using light as an energy source. High rates result in the incorporation into protein of up to 100 nanomoles tritiated leucine per milligram chlorophyll. These rates suggest extensive reinitiation, and repeated utilization of the messenger RNA that code for thylakoid proteins. Up to 39 radioactive thylakoid peptide bands were detected by fluorography after labeling with tritiated leucine.