Dendritic cells (DC) are the most potent antigen-presenting cells (APC); their ability to induce proliferation of T cells in a mixed lymphocyte reaction (MLR) assay is commonly used for the evaluation of their function. It is a general thought that gamma irradiation of APC does not influence their ability to activate T-cell proliferation, but the data from several studies are controversial. To further determine the mechanisms involved in DC-induced T-cell activation in MLR assay, human DC induced from peripheral blood mononuclear cells (PBMC) were gamma-irradiated and determine their effects on the proliferation and cytokine profiles of T cells in an autologous MLR. DC were induced from the PBMC of 11 multiple sclerosis (MS) patients with RMPI 640 medium containing recombinant human GM-CSF (rhGM-CSF; 800 U/ml) and recombinant human IL-4 (rhIL-4; 500 U/ml). DC harvested on day 7 were divided into two equal parts. One part was not irradiated (naive DC); the other was γ-irradiated at a dose of 30 Gy. Cell surface molecules were analyzed by flow cytometry. T-cell proliferation was determined using a beta-scintillation counter. The levels of IL-2, IL-4, IL-6 and IL-10 in co-culture supernatants were measured by ELISA. The results indicated that gamma irradiation reduced expression of CD86, CD80 and HLA-DR molecules on DC, especially CD86 (P=0.0072). DC, irradiated or non-irradiated, effectively stimulated autologous T-cell proliferation. Compared to naive DC, irradiated DC showed a markedly lower capacity to promote T-cell proliferation (P=0.0073), and strikingly up-regulated secretion of IL-4 (P=0.0145) and IL-2 (P=0.0323) by autologous T cells. No significant differences were noted in IL-6 and IL-10 production between T cells co-cultured with naive DC and irradiated DC (P>0.05). It is concluded that gamma irradiation of DC not only influences the phenotype of DC but also alters their capacity to stimulate the proliferation and the cytokine profiles of autologous T cells in a MLR.