Abstract: Previous studies have shown that FasL-expressing cells produced nearly equal amount of soluble FasL (sFasL) and microvesicle-associated FasL (vFasL) under regular tissue culture condition. Here, we studied the ability of TRAIL-expressing cells to produce sTRAIL and vTRAIL and compared their impact on TRAIL-mediated cytotoxicity. We found that TRAIL-expressing cells produced extremely low level of vTRAIL. It indicates that the ability of TRAIL-expressing cells to produce vTRAIL but not sTRAIL is significantly different from FasL-expressing cells and that the ability to produce vTRAIL and vFasL is a property intrinsic to the protein itself. Our study also shows that the microvesicles, containing full-length TRAIL, express strong cytotoxicity against a commonly used Jurkat target cells whereas the cytotoxicity of sTRAIL was nearly undetectable. We concluded that sTRAIL is efficiently produced so much so that it can be inhibitory for the cytotoxicity expressed by the TRAIL-expressing cells. The significance of the findings is discussed.