Nuclear phosphoproteins are labeled with [^(32)p]-phosphate using the tissue culture of HeLa cells. The labeled nuclear proteins were fractionated by DEAE-cellulose chromatography, followed by polyacrylamide gel electrophoresis. There were approximately 120 species obtained which showed the abundance of 10⁴- 10^6 copies/cells. Their structural analyses showed that the simple phosphoproteins contained phosphoamino acids including phosphotyrosine (2 species), phosphothreonine and phosphoserine: and the complex phosphoproteins contained, in addition to phosphoserine, RNA structure (2 species) or poly-ADPR structure (20 species). Patterns of interphase nuclear chromosomal phosphoproteins are different from M phase chromosomal phosphoproteins. Nucleolar protein C23 showed hyperphosphorylation, whereas B23 and p125 kD showed decreased [^(32)p]-association. These are reflected in the nucleolar dissembly. The p 125 kD is the RNA structure containing phosphoprotein which showed marked alterations.