Metrizamide gradients were tested for the possible fractionation of the constitutive units of nuclear particles. Material from 35-55 S monoparticles was indeed distributed along the gradient but rerun experiments, CsCl density determinations, formaldehyde fixation prior to centrifugation suggested that the separation was due to dissociation or (and) action of endogeneous ribonucleases rather than to monoparticle fractionation. That dissociation had indeed occured was confirmed by the study of 60-110 S polyparticles. They were dissociated into ribonucleoproteins rich in phosphoproteins and into free proteins. These products were essentially similar to those obtained after NaCl treatment of the particles though the modes of action of metrizamide and NaCl are likely to be different. The loss of proteins from particles reaches 60-70% and we conclude that metrizamide gradients are not utilizable for the fractionation of the units of nuclear particles.