Several major pathological characteristics of atopic disease are causally related to CD4+ allergen-specific type 2 T-helper (Th2) cells with an aberrant cytokine secretion profile, comprising high levels of interleukin (IL)-4 and IL-5 and low levels of interferon (IFN)-gamma. Although the cytokine secretion patterns of CD4+ T-cells may be stable, they can be modulated by physiological factors which may be expected to be present during activation of these T-cells. In this review, we will focus on two secretion products of professional antigen presenting cells (APCs) and accessory cells with opposite modulatory effects on T-cell cytokine profiles, i.e. prostaglandin E2 (PGE2) and IL-12. PGE2 favours Th2-like cytokine secretion profiles by inhibiting the production of the Th1-associated cytokines, IL-2 and IFN-gamma, and in the presence of sufficient levels of IL-2, upregulating the production of the Th2-associated cytokines, IL-4 and IL-5, IL-12, on the other hand, induces and enhances IFN-gamma secretion in activated CD4+ T-cells, thereby promoting the generation of Th1 cells. PGE2 and IL-12 act via independent mechanisms and, therefore, do not mutually interfere with their modulatory effects. These data suggest that the relative contribution of PGE2 and IL-12 to the levels of secreted Th1- and Th2-associated cytokines are determined by their concentration ratio during T-cell activation.