High ribonuclease B activity associated with chloroplasts is localized in the stroma and the chloroplast membrane, and a negligible amount is associated with mitochondria and ribosomes. The amount of this enzyme retained by the chloroplast is affected by the isolation medium used. Cold treatment enhanced its activity. Ribonuclease A, which is confined to the cytoplasm, is removed only to the extent of 50% by bentonite. Part of ribonuclease A is bound in an inactive complex. Ribonuclease B increases more than A during shoot development. The methylated albumin kieselguhr elution profiles of nucleic acids are affected by ribonucleases. Mainly the ribosomal types of RNA are degraded and this is most pronounced for chloroplasts from older leaves. The two peaks for DNA have been tentatively attributed mainly to chloroplast and nuclear DNA. A considerable amount of nucleic acid, assumed to be mostly mRNA, was bound tenaciously to the column. This fraction decreased with age of leaves. Pulse labeling resulted in two DNA–RNA hybrids which were readily susceptible to endogenous enzyme.