PURPOSE: MIP-1 alpha, initially identified as a substance to promote inflammation, was recently discovered to also suppress proliferation of many kinds of cells. In this study, the influence of MIP-1 alpha on CD4+ Th cells to secrete cytokines (IL-10, IL-2, IFN-gamma) and to express CD25 molecules was investigated. METHODS: Peripheral blood mononuclear cells (PBMCs) were prepared from five normal volunteers and initially divided into 4 groups: IL-10, IL-2, IFN-gamma, and CD25. Each group was further divided into 4 subgroups according to the incubation with or without MIP-1 alpha and to the incubation for 30 minutes or 3 hours. Analysis was performed by flow cytometry. RESULTS: Incubation of CD4+ Th lymphocytes with MIP-1 alpha showed a tendency to increase Th1 cytokine (IL-2, IFN-gamma) secretion and to decrease Th2 cytokine (IL-10) release, but there was no significant difference between any of the experimental groups. Among the CD4+ Th lymphocyte groups cultured with MIP-1 alpha, the expression of CD25 was significantly lower in the 3-hour incubation group than in the 30-minute group (P=0.008). CONCLUSION: MIP-1 alpha may play a role in facilitating immune response by increasing Th1 and decreasing Th2 cytokine secretion from CD4+ Th cells, and also by decreasing the proportion of CD4+CD25+ Th cells in the peripheral blood. However, in vivo study is necessary to verify the function of MIP-1 alpha in the living body.