Fas ligand (FasL) is a type II membrane protein and its biologically active soluble form (sFasL) is made by matrix metalloproteinase cleavage. sFasL may induce apoptosis in the Fas-bearing cells and this special biological activity may be applied to cancer therapy. However, it is difficult to obtain recombinant sFasL because of the trimeric form of the protein. In this study, sFasL DNA was fused to the gene of thioredoxin at the amino-terminal and His-tag at the carboxyl terminal before overexpression in Escherichia coli. To restrict the degree of freedom in the subsequent refolding process, recombinant sFasL was dissolved in 6 M urea and then immobilized with nickel resin under the denaturing condition. The immobilized recombinant sFasL was incubated with excess denatured sFasL to perform refolding process by dilution under stepwise conditions. After the refolding procedures, stable trimers of sFasL oligomers formed and were identified by gel filtration. co-immunoprecipitation and Western blotting. The sFasL showed biological activities in inducing apoptosis in Fas-expressing T cell and glial cell lines. These findings indicate that preparation of trimer forms of sFasL from E. coli is feasible and this strategy may be useful for large-scale production of sFasL. (C) 2004 Elsevier Inc. All rights reserved.