Protein B23 is a major nonribosomal nucleolar protein and putative ribosome assembly factor that has been demonstrated to form oligomers. Sedimentation velocity and equilibrium analyses were used to examine the oligomerization properties of recombinant proteins B23.1 and B23.2. Under low ionic strength conditions protein B23.1 was predominantly a 2.1S monomer with small amounts of a 7.1S oligomer. At NaCl concentrations of 0.1 M and above the protein was almost exclusively the 7.1S oligomer. The oligomer remained the predominant species in NaCl concentrations as high as 1 M, suggesting that oligomers are not stabilized by electrostatic interactions. Low concentrations of divalent metal ions (0.1 - 1mM Ca2+ or Mg2+) also promoted oligomerization. Reducing agents had no effect on oligomerization, indicating that disulfide bridges are not important in oligomer formation. Protein B23.2, the carboxyl-terminal truncated isoform, had sedimentation characteristics similar to that of protein B23.1, suggesting that the carboxyl-terminal end of protein B23.1 is not essential for oligomerization. Protein B23.1 was previously shown to bind nucleic acids [Wang, D., Baumann, A., Szebeni, A., & Olson, M. O. J.(1995) J. Biol. Chem. 269, 30994-30998]. The effect of protein B23.1 oligomerization on its interaction with a 230 base pair DNA fragment was examined by sedimentation analyses. Under conditions where significant amounts of monomer were present, protein B23.1 was capable of binding DNA, whereas conditions that strongly favored oligomerization caused a nearly complete abolition of DNA binding activity. These studies suggest that protein B23 exists in an equilibrium between monomer and oligomer and that the quaternary structure of the protein may regulate its DNA binding properties.