We are investigating the role of gamma-chain in functions mediated by the high affinity Fc receptor for IgG (Fc gamma RI). In a previous study, we found that gamma-chain, which is a member of the family of zeta-chain proteins, associates with Fc gamma RI. Here we show that clustering of Fc gamma RI leads to a rapid and transient tyrosine phosphorylation of gamma-chain in U937 cells. The response was limited to Fc gamma RI activation, and no phosphorylation of gamma-chain was observed after cross-linking of monoclonal antibodies to other surface receptors on these cells. The gamma-chain phosphorylated after Fc gamma RI clustering was the gamma-chain associated with the receptor. We also identified Syk as one of the kinases associated with the receptor complex. Upon Fc gamma RI activation, Syk, but not ZAP-70, was phosphorylated, and reimmunoadsorption experiments of phosphoproteins from immune complex in vitro kinase assays indicated that Syk is part of the activated gamma-chain-Fc gamma RI complex. These results suggest that gamma-chain links Fc gamma RI to intracellular transduction pathways.