Xenopus oocyte-specific poly(A) RNA-binding proteins were isolated and used to prepare monoclonal antibodies. One antibody was used to characterize one particular antigen by immunoblot analysis. The antigen had a molecular weight of 56,000 was oocyte-specific, and decreased in amount during oogenesis. The antigen was localized in the cytoplasm throughout oogenesis and sedimented mainly at 40-60 S. The antigen also was shown to bind poly(A) RNA following chromatography of ribonucleoprotein particles on oligo(dT)-cellulose. The antibody was used to immunoadsorb nontranslating ribonucleoprotein particles. Fifty-five per cent of the poly(A) RNA sedimenting between 40-60 S was shown to be bound by the antigen. The further use of this antibody in attempting to examine other components of the ribonucleoprotein particle is discussed.