Oncogenic forms of the Ras GTPase exhibit defective GTP hydrolase activity and are insensitive to the stimulatory activity of GTPase activating proteins. It has been suggested that a potential therapeutic strategy to inactivate such mutant forms of Ras could involve small molecules that restore GTP hydrolase activity to mutant Ras proteins; however, thus far, such molecules have not been developed. While characterizing the biochemical properties of several commonly detected K-Ras mutants, we made the unexpected observation that an activity in crude bacterial cell extracts was capable of stimulating the conversion of the oncogenic K-RasG13D mutant from a GTP-bound, active form to a GDP-bound, inactive form. The activity was purified, and the protein, nucleoside diphosphate kinase (NDK), was identified as being responsible for the Ras regulating activity. NDK is closely related to the human metastasis suppressor, NM23, which has previously been implicated in regulating the nucleotide state of small GTPases of the Ras family. Although the physiological relevance of such regulation has been controversial, our biochemical findings in in vitro assays indicate that it may be feasible to develop a therapeutic strategy to achieve the selective biochemical inactivation of oncogenic Ras proteins.