T cell antigen receptor (TCR) binding of a peptide antigen presented by antigen-presenting cells (APCs) in the context of surface MHC molecules initiates signaling events that regulate T cell activation, proliferation and differentiation. A key event in the activation process is the phosphorylation of the conserved tyrosine residues within the CD3 chain immunoreceptor tyrosine-based activation motifs (ITAM5), which operate as docking sites for SH2 domain-containing effector proteins. Phosphorylation of the CD3 zeta ITAMs renders the CD3 chain capable of binding the zeta-chain associated protein 70 kDa (ZAP70), a protein tyrosine kinase that is essential for T cell activation. We found that TCR/CD3 crosslinking in Jurkat T cells promotes the association of Crk adaptor proteins with the transiently phosphorylated CD3 zeta chain. Pull down assays using bead-immobilized GST fusion proteins revealed that the Crk-SH2 domain mediates binding of phospho-CD3 zeta. Phospho-CD3 zeta binding is selective and is mediated by the three types of Crk, including CrkI, CrkII, and CrkL, but not by other SH2 domain-containing adaptor proteins, such as Grb2, CRAP and Nck. Crk interaction with phospho-CD3 zeta is rapid and transient, peaking 1 min post TCR/CD3 crosslinking. The results suggest the involvement of Crk adaptor proteins in the early stages of T cell activation in which Crk might help recruiting effector proteins to the vicinity of the phospho-CD3 zeta and contribute to the fine-tuning of the TCR/CD3-coupled signal transduction pathways. (C) 2017 Elsevier Inc. All rights reserved.