As an initial step towards understanding the role of mRNP complexes in translational regulation during compensatory renal hypertrophy, characteristics of polysome-associated mRNP isolated by affinity chromatography were studied. Renal mRNP contained 15-30 percent of the counts after a 1 hr pulse with -3H-orotic acid; it sedimented mainly between 10S and 100S and had a buoyant density of 1.42-1.44 g/cm-3. RNA derived from the mRNP sedimented between 5S and 40S on sucrose density gradients, with the greatest radioactivity in the region of 15S. After labeling with -3H-adenine for 1 hr, up to 17 percent of the radioactivity present in the mRNP-associated RNA was resistant to digestion by pancreatic and T1 ribonucleases. The mRNP protein moiety contained six polypeptides with molecular weights 69,000, 75,000, 80,000, 100,000, 109,000, and 118,000 daltons, which were undetected in the material not binding to oligo(dT)-cellulose.