Interactions between thymocytes and thymic stromal cells are necessary for T cell differentiation, maturation and proliferation. The signals required for these events to occur often necessitate close contact, and indeed adhesion, between the cell types involved. While the transmission of signals from stromal cells to thymocytes has been well documented, there is little evidence that binding of thymocytes to stromal cells can result in stromal cell activation. We have recently identified a novel thymic epithelial adhesion complex composed of three non-covalently associated glycoproteins (gp23, gp45 and gp90). While gp23 and gp45 are jointly required for adhesion to thymocytes, the function of gp90 is unknown. In the present work, we show that gp23/45-mediated contact with thymocytes induces de novo tyrosine phosphorylation of gp90. Furthermore, the protein tyrosine kinase responsible for gp90 neophosphorylation is itself an integral part of the adhesion complex.