Constitutive active M-Ras mutants bind GTP. Thin-layer chromatography of in vivo ³²P-labeled GDP and GTP eluted from M-Ras immunoprecipitated from BAF-3 cells transfected with the indicated experimental plasmids encoding the wild-type or activated mutant M-Ras/22 Val. Same results were obtained with M-Ras/71Lys and M-Ras/22Lys.