We have recently identified the protein product of the c-cbl proto-oncogene as an SH3 binding protein expressed in macrophages. To investigate the possibility that p120c-cbl is involved in signaling pathways initiated by cell surface receptors for IgG (Fc gamma R), lysates of HL60 cells were examined for tyrosine phosphorylation of p120c-cbl upon Fc gamma R engagement. Our findings demonstrate that p120c-cbl is tyrosine-phosphorylated upon Fc gamma R engagement and that this molecule represents the major tyrosine kinase substrate in this signaling pathway. Protein complexes containing p120c-cbl, p72syk, and p56lyn were observed either in resting or activated cells. In vitro studies showed that the direct association between p120c-cbl and p56lyn was mediated by the SH3 domain of p56lyn.