It is known that both interleukin-4 (IL-4) and IL-13 are produced by Th2-type cells and share similar biological functions with each other. However, recently accumulated evidences have revealed that IL-4 may be involved in the Th1-type response. Both thymus and activation-regulated chemokine (TARC/CCL17), a ligand for CC chemokine receptor 4 that is mainly expressed on Th2-type cells, and interferon-induced protein of 10kDa (IP-10/CXCL10), a ligand for CXC chemokine receptor 3 that is mainly expressed on Th1-type cells, are produced by keratinocytes after the stimulation with the primary cytokines such as tumor necrotic factor-alpha (TNF-alpha) and/or interferon-gamma (IFN-gamma). In this study, we investigated the regulation of TARC or IP-10 production from HaCaT cells, an immortalized human keratinocyte cell line, after stimulation with TNF-alpha, IFN-gamma, IL-4 and/or IL-13. Without stimulation, HaCaT cells did not produce TARC. When both TNF-alpha and IFN-gamma were added, they increased synergistically (P<0.003). In addition, when HaCaT cells were stimulated with IL-4, but not IL-13, in combination with TNF-alpha and IFN-gamma, the supernatant TARC levels significantly decreased compared to those with both TNF-alpha and IFN-gamma (P<0.009). This inhibition was completely abolished with the addition of neutralizing anti-IL-4 antibody. The supernatant IP-10 levels also increased synergistically by stimulation with TNF-alpha and IFN-gamma for 24h (P<0.001). When IL-4, but not IL-13, was added to the medium and the cells were co-cultured with these cytokines, the IP-10 levels significantly increased compared to those with both TNF-alpha and IFN-gamma (P<0.04). Furthermore, the effects of IL-4 on TARC and IP-10 production in these cells were detected in a dose-dependent manner. These data strongly suggest that IL-4 may act not only as a mediator of Th1-type response but also as a down-regulator of Th2-type response in terms of the regulation of chemokine production by HaCaT cells.