The binding of synthetic polyribonucleotides to ribosomes has served as a useful model system for the elucidation of some of the facets of the binding of natural messenger RNA. In this work, certain quantitative aspects of the binding of polyuridylic acid to ribosomes were studied. Polyacrylamide gel filtration was used to isolate polyuridylic acid-ribosome complexes. The experiments indicate that in the process of formation of a polyuridylic acid-ribosome complex, the longer polymer chains are preferentially bound. Therefore, meaningful quantitative evaluation of the stoichiometry of polyuridylic acid-ribosome binding should take into account not only the number of polyuridylic acid chains bound but also the size of the chains. © 1969.