Cdc42 plays an important role in intracellular signaling pathways that influence cell morphology and motility and stimulate DNA synthesis. In attempts to determine whether nonreceptor tyrosine kinases play a fundamental role in Cdc42 signaling, we have cloned and biochemically characterized a new Cdc42-associated tyrosine kinase (ACK) from bovine brain. This tyrosine kinase, named ACK-2, has a calculated molecular mass of 83 kDa and shares a number of primary structural domains with the 120-kDa ACK (ACK-1). The main differences between the primary structures of ACK-2 and ACK-1 occur in the amino- and carboxyl-terminal regions. Like ACK-1, ACK-2 binds exclusively to activated (GTP-bound) Cdc42 and does not bind to its closest homologs, e.g. activated Rac. ACK-2 could not be activated by addition of glutathione S-transferase (GST)-Cdc42(Q61L), a GTPase-defective mutant, or by GTPgammaS-loaded GST-Cdc42 in in vitro kinase assays. However, ACK-2 was activated when cotransfected with wild type Cdc42 or Cdc42(Q61L) and stably associated with Cdc42(Q61L) in vivo, indicating that ACK-2 interacts with active Cdc42 in cells. Furthermore, the tyrosine kinase activity of ACK-2 was stimulated both by epidermal growth factor and bradykinin, suggesting that ACK-2 may play a role in the signaling actions of both receptor tyrosine kinases or heterotrimeric G-protein-coupled receptors.