The Syk protein tyrosine kinase (PTK) is expressed in many hematopoietic cells and is involved in signaling from various receptors for antigen and Fc portions of IgG and IgE. Upon cross-linking of these receptors, Syk is rapidly phosphorylated on tyrosine residues and enzymatically activated. We and others have found that the Lck kinase, a member of the Src family of PTKs, binds through its Src homology (SH) 2 domain to tyrosine phosphorylated Syk and to the related Zap kinase. Here we report that this interaction is direct and identify the two tandem tyrosines at the autophosphorylation site of Syk, Tyr518, and Tyr519, as the binding site for the SH2 domain of Lck. Mutation of either or both tyrosines to phenylalanines abrogated binding, while mutation of a second repetition of the motif at Tyr539 and Tyr540, or of the three tyrosines in the C terminus of Syk, did not. The SH2 domain of Lck bound the autophosphorylation site only when both Tyr518 and Tyr519 were phosphorylated. In intact cells the binding of the SH2 domain of Lck correlated with the ability of Syk to induce tyrosine phosphorylation of cellular proteins.