Nucleoside diphosphate kinase from the cytosol of rat brain was purified to electrophoretic homogeneity through a series of chromatography columns, including DE-52, AcA-34 Ultrogel, phenyl-Sepharose and Q-Sepharose. The molecular weight of the enzyme was 64,000 as estimated by gel filtration, and it consisted of 3 identical 19,000 subunits. [35S]-GTP gamma S binding and autophosphorylation of the enzyme, and inhibition of the GTPase activity of G-protein were used to screen for the enzyme. These methods were shown to be useful for purifying and assaying the enzyme.