Ras -related small GTPases serve as critical regulators for a wide range of cellular signaling pathways and are activated by the conversion of the GDP-bound state to the GTP-bound conformation. Until recently, measurement of the GTP-bound active form of Ras -related G proteins involved immunoprecipitation of 32 P-labeled protein followed by separation of the labeled GTP/GDP bound to GTPase. A new method based on the large affinity difference of the GTP- and GDP-bound form of Ras proteins for specific binding domains of effector proteins in vitro has been developed. By using glutathione S -transferase (GST) fusion proteins containing these binding domains, the GTP-bound form of the GTPase can be precipitated from cell lysates. In principle, this method can be used for all members of the Ras superfamily. Here we describe a general procedure to monitor the GTP-bound form of Ras -related GTPases.