From the cloned heavy and light chains of a murine monoclonal antibody (mAbB23) which is specific for human apolipoprotein (apo) B-100 of plasma low-density lipoproteins, a vector was designed for expression of a single-chain antibody (scFv) of mAbB23 in Escherichia coli. The expression vector was constructed so that the scFv gene (V-L-linker-V-H) was expressed under the control of the T7 promoter. The inclusion body of scFv was isolated from E. coli lysate and solubilized in 6 M guanidine-hydrochloride without reducing agents, followed by refolding through slow dilution into refolding buffer. After complete removal of the remaining denaturant by dialysis, the soluble scFv was purified through an apo B-100-coupled affinity column, and an active fraction, which had an antigen-binding activity comparable with that of native Fab, was easily obtained. The expression and in vitro refolding of scFv resulted in production of an active molecule in a yield of 15-20 mg per 1-liter flask cultivation. (C) 2002 Elsevier Science (USA).