A discrete set of polypeptides copurify with and appear to be specifically attached to mRNA from polysomes of eukaryotic cells. This report describes the effect of homopolyribonucleotides on mRNA-protein complexes separated from ribosome subunits by oligo(dT)-cellulose chromatography. It is shown that poly (U) and poly (A) can release mRNA-protein complexes adsorbed to oligo(dT)-cellulose, whereas poly (C) and poly(I) are much less effective in this process. Analysis of polyribonucleotide released material showed that poly(U) effectively dissociated the mRNA-protein complexes while poly(A) caused no or only partial derangement of these particles. The specificities seen in the polyribonucleotide effects in turn suggest a high degree of specificity in the interaction between the proteins and mRNA.