Fructose bisphosphatase-1 (FBP1) is a rate limiting enzyme in gluconeogenesis that converts fructose-1,6-bisphosphate (F1,6BP) to fructose-6-phosphate (F6P). It is active in liver, kidney and skeletal muscle cells. This study suggests that FBP1 plays a novel non-gluconeogenic role in T cells. Targeted metabolomics using [13C]-6-glucose revealed a labeling pattern of F6P in stimulated CD3+ T cells that could only have resulted from FBP1 enzymatic activity. Following stimulation, T cells expressed a 27kD form of FBP1, in addition to the full-length 37kD protein. The hypothesis that T cells utilize an alternative translational start site to express a shorter, constitutively active, form is being tested. Future studies will also test the hypothesis that FBP1 activity increases carbon flux into the pentose phosphate pathway (PPP), to facilitate increased production of reducing agent and co-factor, NADPH, in preparation for proliferation. This research could contribute significantly to our understanding of T cell physiology and cancer cell metabolism.