Dendritic cells (DCs) have been shown to express the tryptophan catabolizing enzyme indoleamine 2,3-dioxygenase (IDO-1), a protein presently thought to exert dual and possibly contrasting effects on the immune response. Depletion of tryptophan and release of tryptophan catabolites have been shown to exert a tolerogenic influence on T cell responses, while the IDO enzymatic activity has been recently suggested to promote DC maturation. In this report, we have explored the putative role of IDO-1 in regulating DC biology by analyzing DC development and function from IDO-1 deficient mice. In keeping with previous observations, lack of IDO-1 expression was found to affect in vitro DC generation from bone mouse precursors cultured in the presence of GM-CSF. However, change in growth factor (Flt3L) and/or culture conditions (low-adherence vessels) abolished the difference observed between wt (wild type) and IDO-1-deficient, in vitro generated DCs. Moreover, IDO-1-deficient mice displayed a normal DC compartment in vivo, suggesting that IDO-1 does not play a significant role in DC development and function in vivo. Collectively, these observations suggest that despite a possible role for IDO-1 expression in regulating DC differentiation in vitro under commonly used culture conditions, IDO-1 is largely dispensable for DC development and function in vivo.