ABSTRACT Nucleoplasmin is a phosphorylated nuclear-accumulating protein. We report herein that the kinetics of its cytoplasm r nucleus transport are affected by its degree of phosphorylation. Therefore, we sought to identify any protein kinase which specifically associates with nucleoplasmin. We discovered that nucleoplasmin co-isolates by two independent methods (immunoabsorption and chromatography) in a complex including a kinase which phosphorylates nucleoplasmin. The co-purifying kinase is casein kinase II-like because: (i) it phosphorylates casein; (ii) its phospho-transferase activity can be competed out by GTP; (iii) it is stimulated by polylysine; and (iv) it is inhibited by heparin. Moreover, a polyclonal antibody to the α (38 kDa) and α′ (36 kDa) catalytic subunits of casein kinase II specifically recognizes 38 and 36 kDa polypeptides in the nucleoplasmin-complex, and a specific inhibitor of casein kinase II inhibits nucleoplasmin’s nuclear transport. Additionally, we found that phosphorylation of nucleoplasmin by its associated casein kinase II is strongly inhibited by histones and that, in addition to nucleoplasmin, another protein (p100) in the nucleoplasmin-complex is phosphorylated by casein kinase II.