The Bcr/Abl oncoprotein is directly responsible for the development of chronic myelogenous leukemia and Philadelphia chromosome-positive acute lymphoblastic leukemia in humans. The adapter protein Crkl is one of the most prominently tyrosine-phosphorylated substrates of Bcr/Abl in cells and tissues isolated from such patients. The guanine nucleotide exchange factor for the small GTPase Rap1, C3G, binds constitutively to Crkl. Here, we report that Crkl mediates the formation of protein complexes that include C3G and Bcr/Abl. These complexes contain highly elevated levels of tyrosine-phosphorylated C3G and P130Cas, a scaffolding protein. Moreover, the presence of Rap1 further promoted tyrosine phosphorylation of C3G and Cas. Co-expression of Crkl and C3G with Bcr/Abl generated increased levels of activated Rap1. In addition, lysates from leukemic cells of P190 BCR/ABL transgenic mice and of the myelogenous leukemia cell line K562 contained tyrosine-phosphorylated C3G and activated Rap1. These data suggest a role for C3G-mediated Rap1 activation in Bcr/Abl-induced leukemia development.