The present study aimed to investigate the effect of diallyl trisulfide (DATS) on tumor necrosis factor (TNF)-alpha expression in inflammed mucosa of ulcerative colitis and its possible mechanism. Colonic biopsies from ulcerative colitis were treated with 0, 1, 5, and 10 microM DATS for 24 hr. Lactate dehydrogenase (LDH) activities and concentrations of TNF-alpha in supernatants were measured. mRNA expressions of TNF-alpha in biopsies were analyzed by RT-PCR. The expression of NF-kappaB P65 in tissues was studied by immunohistochemistry. Concentrations of TNF-alpha in supernatants of biopsies treated with 5 and 10 microM DATS were lower than those of untreated biopsies. There were fewer lamina propria mononuclear cells whose NF-kappaB P65 expression in nuclei was positive, and less mRNA expression of TNF-alpha in biopsies treated with 10 microM DATS than in untreated biopsies. There were no differences in LDH activities in supernatants between tissues treated with DATS and untreated tissues. DATS could downregulate TNF-alpha production and inhibit NF-kappaB activation in lamina propria mononuclear cells of inflammed mucosa, without any effect on the viability of colonic tissue cells.