The Wiskott-Aldrich syndrome (WAS) is a rare immunodeficiency disease affecting mainly platelets and lymphocytes. Here, we show that the WAS gene product, WASp, is tyrosine phosphorylated upon aggregation of the high affinity IgE receptor (Fc epsilonRI) at the surface of RBL-2H3 rat tumor mast cells. Lyn and the Bruton's tyrosine kinase (Btk), two protein tyrosine kinases involved in Fc epsilonRI-signaling phosphorylate WASp and interact with WASp in vivo. Interestingly, expression of a GTPase defective mutant form of CDC42, that interacts with WASp, is accompanied by a substantial increase in WASp tyrosine phosphorylation. This study suggests that activated CDC42 recruits WASp to the plasma membrane where it becomes phosphorylated by Lyn and Btk. We conclude that WASp represents a connection between protein tyrosine kinase signaling pathways and CDC42 function in cytoskeleton and cell growth regulation in hematopoietic cells.