Enhancement of Syk, but not ZAP-70, kinase activity in response to FcγRI stimulation. U937, U937/LN, U937/ZAP-70, and Jurkat cells were subjected to antibody-mediated cross-linking followed by immunoprecipitation and analysis of endogenous substrate phosphorylation. Syk (a) and ZAP-70 (b) activities in immunoprecipitates from nonstimulated (−) or cross-linked (+) cells were assessed by immune complex kinase assay and autoradiography. These data are representative of results obtained in five independent experiments.