Ribonucleoprotein fragments were isolated from Ehrlich ascites cell messenger ribonucleoprotein (mRNP), immobilized on oligo(dT)-cellulose, by successive RNAase and formamide treatments and their RNA and protein moieties were characterized. A 8--10 S formamide-eluted (F) RNP primarily contained the poly(A) segment as an RNA moiety, as evidenced by its characteristic electrophoretic mobility, resistance to RNAase digestion, affinity for nitrocellulose filters and poly(U), and base composition. In contrast, the RNAase-eluted (R) fraction was composed of a 2--3 S RNP carrying a much shorter polyribonucleotide fragment of heterogeneous base composition and properties unlike poly(A). Sodium dodecyl sulfate-polyacrylamide electrophoresis indicated that the protein moieties of the R fraction were composed primarily of 56 000, 67 000, and 71 000 molecular weight polypeptides whereas those of the F fraction contained the latter three polypeptides as well as a major species exhibiting a molecular weight of 81 000. The data demonstrate that RNA sequences other than poly(A) interact with protein in mRNP, that three major mRNP polypeptides are associated with both poly(A) and non-poly(A) RNA binding sites, and that one major polypeptide is specifically bound to the poly-(A) region of mRNP.