Unlike the alpha subunits of heterotrimeric guanosine triphosphate (GTP)-binding proteins, Ras-related GTP-binding proteins have hitherto been considered not to bind or become activated by tetrafluoroaluminate (AlF4-). However, the product of the proto-oncogene ras in its guanosine diphosphate (GDP)-bound form interacted with AlF4- in the presence of stoichiometric amounts of either of the guanosine triphosphatase (GTPase)-activating proteins (GAPs) p120(GAP) and neurofibromin. Neither oncogenic Ras nor a GAP mutant without catalytic activity produced such a complex. Together with the finding that the Ras-binding domain of the protein kinase c-Raf, whose binding site on Ras overlaps that of the GAPs, did not induce formation of such a complex, this result suggests that GAP and neurofibromin stabilize the transition state of the GTPase reaction of Ras.