The effect of actinomycin D on the synthesis of the photosynthetic apparatus during illumination of etiolated leaves of Phaseolus vulgaris was studied. The increase of chlorophyll content and of the activities of some photosynthetic enzymes (NADPH diaphorase, ferredoxin, NADP(+) glyceraldehyde-3-phosphate dehydrogenase) was compared with simultaneous measurements of the level of other enzymes not considered associated with photosynthesis (ornithine transcarbamylase, glucose-6-phosphate dehydrogenase, NAD(+) glyceraldehyde-3-phosphate dehydrogenase).The effect of the inhibitor on the synthesis of the components of the photosynthetic apparatus is much larger than its effect on the synthesis of non-photosynthetic enzymes when the antibiotic is supplied 2 hr before illumination. The same selective action is also obtained if actinomycin D is added after 20 hr of exposure of the leaves to light.The markedly different sensitivity to the inhibitor of the synthesis of photosynthetic enzymes, as compared to non-photosynthetic ones, is interpreted as a selective inhibition at the level of DNA-directed synthesis of RNA molecules.This RNA may be involved either in the regulation of chloroplast differentiation or in the specification of some component essential for the formation of the plastidial structure or for the activity of plastidial ribosomes.