Fas ligand (FasL) is a type II membrane protein. Binding of FasL to its receptor, Fas, induces apoptosis. Matrix metalloproteinase cleaves the membrane-bound human FasL to yield the active soluble form. Here, we have produced a large amount of human soluble rFasL using the yeast, Pichia pastoris. The purified rFasL was found to be glycosylated and to exist as a trimer. The rFasL was effective in inducing apoptosis in a Fas-expressing T cell or a fibroblast cell line. The ID50 of rFasL for mouse Fas-expressing T cells was about 0.5 ng/ml. The killing process with rFasL was quick. That is, >80% Fas-expressing mouse cells were killed within 1 h by a saturation concentration of human rFasL. Intravenous administration of 500 microg of human rFasL had a lethal effect in mice. When the mice were pretreated with Propionibacterium acnes, the subsequent injection of 30 microg of human rFasL induced hepatic failure and killed the mice within 24 h. These results indicated that the soluble human FasL is active in inducing apoptosis in vitro and in vivo, and its deleterious effect may be strengthened in patients who are suffering from bacterial infection.