Recent evidence has shown that H4biopterin is synthesized in cells which undergo cytokine-directed proliferation and do not use H4biopterin as a hydroxylation cofactor. H4biopterin, in turn, enhances the proliferation of erythroleukemic cells and modulates various aspects of the interleukin-2-induced clonal expansion of T cells.1 In these cells the activity of sepiapterin reductase (SR) is in the range of 100 pmol min-1 mg-1.Lectin Stimulation of resting T cells causes a slowly progressing increase, starting from levels below detection and a transient enhancement of SR activity is found after treatment of activated T cells by interferon-γ plus interleukin-2.1 The NK-like human cell line YT and the murine erythroleukemic cell line B8/3 lack any SR activity. The molecular basis of SR regulation has not been explained in any of these cases.