Dendritic cells (DC) are key initiators of primary immune responses. Myeloid DC can secrete IL-12, a potent Th1-driving factor, and are often viewed as Th1-promoting APC. Here we show that neither a Th1- nor a Th2-inducing function is an intrinsic attribute of human myeloid DC, but both depend on environmental instruction. Uncommitted immature DC require exposure to IFN-gamma, at the moment of induction of their maturation or shortly thereafter, to develop the capacity to produce high levels of IL-12p70 upon subsequent contact with naive Th cells. This effect is specific for IFN-gamma and is not shared by other IL-12-inducing factors. Type 1-polarized effector DC, matured in the presence of IFN-gamma, induce Th1 responses, in contrast to type 2-polarized DC matured in the presence of PGE2 that induce Th2 responses. Type 1-polarized effector DC are resistant to further modulation, which may facilitate their potential use in immunotherapy.