The D protein (16 kDa) is part of a protein core, common to U1, U2, U5, U4/U6 small nuclear RNA containing ribonucleoprotein particles. Monoclonal antibodies reactive with the D protein were used in quantitative dot blotting and Western blotting to demonstrate that this protein was a component of salt resistant nuclear structures and was enriched greater than 3 to 5-fold in RNAase-protected nuclear matrix preparations.