PI 3-kinases have been implicated in a wide range of cellular functions in T lymphocytes by experiments employing pharmacologic or genetic approaches to oppose the lipid kinase catalytic function. These experiments however, do not yield data that distinguish between lipid products of PI 3-kinase. Although the major lipid product of PI 3-kinases is PtdIns(3,4,5)P3, levels of PtdIns(3,4)P2 also rise within T lymphocytes following cellular activation [1]. The pleckstrin homology domains of some proteins can bind these lipids separately, for example the tandem PH domain protein (TAPP) 1 will bind PtdIns(3,4)P2 but not PtdIns(3,4,5)P3 [2]. This observation suggests that there may be distinct functions for different phosphoinositides. To explore the biology of PtdIns(3,4)P2 we have fused the TAPP1 PH domain to fluorescent reporters such as GFP and RFP to create a lipid specific bioprobes. We have expressed RFP-TAPP in T cell lines and human primary CD4+ T cells to image the distribution of PtdIns(3,4)P2 within cells, responding to different activation stimuli. Further we have placed RFP-TAPP under the control of the mouse Vav promoter to create transgenic mice expressing RFP-TAPP in their T cell compartment. These studies should enable non-invasive, real-time imaging of PtdIns(3,4)P2 in live cells responding to physiological stimuli.