YB-1 is a universal major protein of cytoplasmic mRNPs, a member of the family of multifunctional cold shock domain proteins (CSD proteins). Depending on its amount on mRNA, YB-1 stimulates or inhibits mRNA translation. In this study, we have analyzed complexes formed in vitro at various YB-1 to mRNA ratios, including those typical for polysomal (translatable) and free (untranslatable) mRNPs. We have shown that at mRNA saturation with YB-1, this protein alone is sufficient to form mRNPs with the protein/RNA ratio and the sedimentation coefficient typical for natural mRNPs. These complexes are dynamic structures in which the protein can easily migrate from one mRNA molecule to another. Biochemical studies combined with atomic force microscopy and electron microscopy showed that mRNA-YB-1 complexes with a low YB-1/mRNA ratio typical for polysomal mRNPs are incompact; there, YB-1 binds to mRNA as a monomer with its both RNA-binding domains. At a high YB-1/mRNA ratio typical for untranslatable mRNPs, mRNA-bound YB-1 forms multimeric protein complexes where YB-1 binds to mRNA predominantly with its N-terminal part. A multimeric YB-1 comprises about twenty monomeric subunits; its molecular mass is about 700 kDa, and it packs a 600-700 nt mRNA segment on its surface.