Within seconds of T cell receptor engagement, a well-characterized series of tyrosine phosphorylation events mediate cellular activation. It is widely accepted that these initial phosphorylations remain stable until protein tyrosine phosphatases return the cell to its basal level. Based on a model of peripheral blood T cell activation, in which the kinetics of phosphorylation can be modulated, we propose an alternate hypothesis that T cell signaling is highly dynamic. Our results demonstrate that tyrosine phosphorylation and dephosphorylation are occurring co-temporally after T cell receptor cross-linking, regulated by a delicate balance of kinases and phosphatases. (C) 2002 Elsevier Science (USA). All rights reserved.