While much is known about the signalling pathways within lymphocytes that are triggered during activation, much less is known about how the various cell surface molecules on T cells initiate these events. To address this, we have focused on the primary interaction that drives T–cell activation, namely the binding of a particular T–cell receptor (TCR) to peptide–MHC ligands, and find a close correlation between biological activity and off–rate; that is, the most stimulatory TCR ligands have the slowest dissociation rates. In general, TCRs from multiple histocompatibility complex (MHC) class–II–restricted T cells have half–lives of 1–11s at 25°C, a much narrower range than found with antibodies and suggesting a strong selection for an optimum dissociation rate. TCR ligands with even faster dissociation rates tend to be antagonists. To observe the effects of these different ligands in their physiological setting, we made gene fusions of various molecules with green fluorescent protein (GFP), transfected them into the relevant lymphocytes, and observed their movements during T–cell recognition using multicolour video microscopy. We find that clustering of CD3ζ–GFP and CD4–GFP on the T cell occurs concomitantly or slightly before the first rise in calcium by the T cell, and that various GFP–labelled molecules on the B–cell side cluster shortly thereafter (ICAM–1, class II MHC, CD48), apparently driven by T–cell molecules. Most of this movement towards the interface is mediated by signals through the co–stimulatory receptors, CD28 and LFA–1, and involves myosin motors and the cortical actin cytoskeleton. Thus, we have proposed that the principal mechanism by which co–stimulation enhances T–cell responsiveness is by increasing the local density of T–cell activation molecules, their ligands and their attendant signalling apparatus. In collaboration with Michael Dustin and colleagues, we have also found that the formation and stability of the TCR–peptide–MHC cluster at the centre of the interaction cap between T and B cells is highly dependent on the dissociation rate of the TCR and its ligand. Thus, we are able to link this kinetic parameter to the formation of a cell surface structure that is linked to and probably causal with respect to T–cell activation.