Rat liver nuclear ribonucleoprotein particles were prepared by two different methods and defined as 40S ribonucleoprotein (40S RNP) and heterogeneous nuclear ribonucleoprotein (HnRNP) particles. The RNP particles were either solubilized in 8 M urea--6 mM 2-mercaptoethanol--20 mM glycine--20 mM Tris--HCl (pH 8.4) or subjected to removal of RNA by phenol extraction prior to solubilizing the proteins in the urea buffer. The proteins associated with 40S RNP and HnRNP were heterogeneous and very similar in their electrophoretic patterns when analyzed by two-dimensional PAGE, except a protein with molecular weight of 62 000 and an isoelectric point (pI) of 6.2 was present only in HnRNP particles. At least 12 major and 22 minor components could be identified in both preparations. The major proteins were found at pI values varying from 6.0 to 8.5 and with molecular weights from 32 000 to 42 000, and a group of proteins with molecular weight approximately 65 000 were more prominent in HnRNP than in 40S RNP. The other components were found mainly at pI ranges from 5.0 to 6.5 with molecular weights from 43 000 to 65 000. The phenol method extracted essentially all proteins associated with either 40S RNP and HnRNP, but was less effective in extracting a group of proteins with pI values from 5.0 to 5.5 and more efficient for proteins with pI values from 7.5 to 8.5. When chromatin proteins isolated by phenol extraction were compared with HnRNP particle proteins isolated by the same method, the electrophoretic mobilities of the HnRNP particle proteins were found to be identical with a fraction nonhistone chromatin proteins. The 40S RNP particles were further purified by metrizamide isopycnic density gradient centrifugation. The electrophoretic patterns of these proteins were very similar to those prepared by sucrose density gradient centrifugation. Therefore, we concluded that the proteins of RNP particles constituted part of the chromatin proteins.