Biosynthesis of tetrahydrobiopterin starts from guanosine triphosphate by the action of guanosine triphosphate cyclohydrolase I, which yields the first intermediate, 7,8-dihydroneopterin triphosphate. This compound is then converted by subsequent enzymes, 6-pyruvoyl tetrahydropterin synthase and sepiapterin reductase, to tetrahydrobiopterin, the biologically active metabolite. Cytokines such as γ-interferon or tumor necrosis factor-α strongly stimulate the activity of guanosine triphosphate cyclohydrolase I in murine and human cells, yielding a potentiation of intracellular tetrahydrobiopterin concentrations. In human cells, particularly in human monocytes and macrophages, the low activity of 6-pyruvoyl tetrahydropterin synthase leads to the additional accumulation of neopterin derivatives, which leak from the cells after dephosphorylation and are found increased in body fluids of humans with diseases challenging cell-mediated immunity. A functional role for the stimulation of tetrahydrobiopterin biosynthesis by cytokines is the formation of a limiting cofactor required for the enzymatic conversion of L-arginine to citrulline and nitric oxide.