Despite extensive study, several of the major components involved in T cell receptor–mediated signaling remain unidentified. Here we report the cloning of the cDNA for a highly tyrosine-phosphorylated 36–38 kDa protein, previously characterized by its association with Grb2, phospholipase C-γ1, and the p85 subunit of phosphoinositide 3-kinase. Deduced amino acid sequence identifies a novel integral membrane protein containing multiple potential tyrosine phosphorylation sites. We show that this protein is phosphorylated by ZAP-70/Syk protein tyrosine kinases leading to recruitment of multiple signaling molecules. Its function is demonstrated by inhibition of T cell activation following overexpression of a mutant form lacking critical tyrosine residues. Therefore, we propose to name the molecule LAT—linker for activation of T cells.