The t(9;22) chromosomal translocation is found in almost all patients with chronic myelogenous leukemia. The resultant Bcr-Abl fusion gene expresses a chimeric fusion protein p210(bcr-abl) with increased tyrosine kinase activity. Hematopoietic progenitors isolated from chronic myelogenous leukemia patients in the chronic phase contain constitutively tyrosine-phosphorylated p62(dok) protein. p62(dok) associates with the Ras GTPase-activating protein (RasGAP), but only when p62(dok) is tyrosine phosphorylated. Here we have investigated the interaction between p62(dok) and RasGAP and the consequences of p62(dok) tyrosine phosphorylation on the activity of RasGAP. We have found that p62(dok) is directly tyrosine phosphorylated by p210(bcr-abl), and the sites of phosphorylation are located in the C-terminal half of the p62(dok) molecule. We have identified five tyrosine residues that are involved in in vitro RasGAP binding and have found that tyrosine-phosphorylated p62(dok) inhibits RasGAP activity. Our results suggest that p210(bcr-abl) might lead to the activation of the Ras signaling pathway by inhibiting a key down-regulator of Ras signaling.