FcεRI is suspected to play a pivotal role in the pathophysiology of atopic disorders such as atopic dermatitis. In search for genes differentially regulated by FcεRI on APCs, a differential cDNA bank of receptor-stimulated and unstimulated monocytes was established. By means of suppression subtractive hybridization, we identified kynurenine 3-monooxygenase and subsequently indoleamine 2,3-dioxygenase (IDO) to be overexpressed in FcεRI-activated monocytes. IDO is the rate-limiting enzyme in the catabolism of the essential amino acid tryptophan. We show that cross-linking of FcεRI on monocytes results in low tryptophan concentrations associated with impaired T cell stimulatory capacity. Importantly, T cell suppression could be prevented by the addition of tryptophan or inhibition of IDO. Moreover, stimulation of T cells by FcεRI-activated monocytes was increased compared with T cell stimulation by nonactivated monocytes if exogenous supply of tryptophan was available. We speculate that the expression of IDO by FcεRI+ APCs in vivo allows these cells to regulate T cell responses in atopic disorders by inhibiting or stimulating T cell proliferation, depending on the metabolic environment.