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Rat liver polysomes, single ribosomes, EDTA-produced subparticles and free ribosomal RNA were treated with formaldehyde and their properties were studied. The following results were obtained. 1. RNA can be isolated from fixed polysomes or subparticles by exhaustive digestion with pronase. Such RNA shows a normal electrophoretic behaviour in agar gel. 2. In free RNA treated with formaldehyde the mobility of 18-S and 28-S RNA fractions is reduced; latent breaks are not revealed by heating; sensitivity towards ribonuclease is increased. No such effects are observed with RNA treated with formaldehyde while in the particles. 3. In fixed polysomes or single ribosomes the two subparticles appear to be covalently bound through their proteins. No intermolecular bonding occurs between 18-S and 28-S RNA either fixed polysomes or in free RNA. The fixed ribosomes and subparticles can be fractionated by agar gel electrophoresis and give profiles similar to those of untreated particles. These resets may prove useful in combining CsCl density gradient fractionation with gel electrophoretic methods. [ABSTRACT FROM AUTHOR]