Abstract: The methods of the disclosure provide fluorescence-based assays for calcineurin activity, especially in isolated T cells. The methods include the stimulation of the T cells with agents that specifically target the TCR with or without influencing co-stimulatory pathways. One TCR agonist is monoclonal antibodies specific for CD3, which more precisely distinguish the inducible activity of calcineurin than does an alternative method targeting the T cell receptor (CD3) combined with CD28 costimulation. This method more accurately distinguishes between the measured level of calcineurin activity of T cells from immunosuppressed transplant recipients and normal individuals, and thus has improved diagnostic accuracy with respect to the response of an individual to immunosuppressant therapy following an organ transplant.