Heterogeneous Nuclear Ribonucleoprotein K (hnRNP K) is an RNA/DNA-binding protein involved in many processes that regulate gene expression. K protein's pleiotropic action reflects the diversity of its molecular interactions. Many of these interactions have been shown to be regulated by phosphorylation. K protein contains more than seventy potential phosphorylation sites. We used an integrated approach of mass spectrometry and computer analysis to explore patterns of K protein phosphorylation. We found that in vitro a single kinase can phosphorylate K protein on multiple sites spanning the entire length of the protein, including residues contained within the RNA/DNA-binding domains. 2-D gel electrophoresis of K protein purified from cells identified 5-8 spots. Mass spectrometry of K protein isolated from proliferating cells and from cells under oxidative stress revealed the same pattern of phosphopeptides. The structural implications of phosphorylation are discussed.