We report the initial characterization of pp64, a nuclear protein recognized by anti-PDGF antisera and uniquely phosphorylated when intact NRK fibroblasts were exposed to PDGF. Neither DNase nor RNase released pp64 from nuclei and pp64 was only partially extracted from nuclei of NRK cells with NaCl; detergents were required for complete extraction. The estimated half-life of pp64 from normal and transformed NRK cells was approximately 6 hrs. The rapid and unique PDGF stimulated phosphorylation of pp64 identifies a previously uncharacterized nuclear phosphoprotein, and a potentially novel signal transduction pathway from the cell surface to the nucleus which may be important in initiating nuclear events associated with DNA synthesis and cell division.