Messenger ribonucleoprotein particles were isolated from the polysomes of logarithmically growing plant cell cultures, pulse-labeled with [3H] adenosine for 30 min. More than 80% of the labeled RNP was present in particles sedimenting between 80 S and 30 S on sucrose density gradients, but was not associated with ribosomal subunits. The size distribution differs from those reported for polysomal mRNP particles to date. After fixation with glutaraldehyde the labeled RNP particles had a buoyant density of 1.38 g/cm3 in CsCl gradients. Radioactively labeled RNA extracted from the RNP particles showed a heterodisperse size distribution and contained poly (A) stretches as determined by affinity chromatography and ribonuclease digestion experiments. © 1978, Walter de Gruyter. All rights reserved.