By measuring the relative amounts of high-molecular-weight ribonueleic acids in chloroplasta and in cytoplasm reliable values were obtained for the purity of chloroplasts isolated in non-aqueous media from leaves of tobacco (Nicotiana tabacum, var. White Barley), broad bean (Viciafaba, var. White Fan), and tomato plants (Lycopersicon esculenium, var. Money Maker). Measurements of pyruvate kinase activity, previously used to test chloroplast purity, agreed well with the results of ribosomal ribonucleic-acid analysis for the bean and tomato leaves. The purest chloroplast fractions from tobacco leaves always contained more pyruvate kinase than could be accounted for on the basis of the cytoplasmic contamination measured by the nucleic-acid analysis. Some pyruvate kinase may therefore be present in the chloroplasts in tobacco leaves. The purest chloroplasts obtained from any of the three species still contained 11 per cent of the cytoplasm even after severe mechanical treatments designed to remove cytoplasm adhering to the surface of the plastids. Chloroplast fractions obtained by the usual non-aqueous techniques always contained at least 15 per cent of the cytoplasm.