Thy-1, a glycosylphosphatidylinositol (GPI)-anchored glycoprotein expressed at high levels on thymocytes, has been implicated in positive and negative signal transduction. We show that Thy-1 associates with a protein of 85–90kDa, which is prominently phosphorylated in vitro as well as in vivo following the stimulation of thymocytes with pervanadate. pp85–90 is not identical to known proteins that are phosphorylated following T cell activation. The SH2 domains of fyn, csk, phosphatidylinositol 3′-kinase, rasGAP, vav and lck bind to pp85–90 with varying affinities. The SH2 domains of ZAP70, SHP-1 and PLCγ1 and the SH3 domains of lck, vav and HS1 did not bind to pp85–90. The molecular weight, iso-electric point, efficient phosphorylation by fyn and lck and preferential binding to the SH2 domain of fyn compared to that of lck indicate that Thy-1-associated pp85–90 may be identical to a recently cloned, fyn-associated transmembrane adaptor protein, PAG-85.