The Pichia pastoris expression system was used to produce functionalized single-chain antibody fragments (scFv) directed against the ED-B domain of the B-fibronectin (B-Fn) isoform which was found to be present only in newly formed blood vessels during tumor angiogenesis, Therefore, scFv antibody fragments recognizing the ED-B domain are potential markers for angiogenesis, We constructed four functionalized scFv antibody fragments for direct labeling with radioactive molecules or toxins or for attachment to liposomes serving as carriers for cytotoxic or antiangiogenic compounds. The C-termini of the scFv antibody fragments contain 1-3 cysteine residues that are separated by a hydrophilic linker (GGSSGGSSGS) from the binding domain and are accessible for site-specific functionalization with thiol-reactive reagents. Plasmid expression, culture conditions, and purification were optimized in l-L cultures. The scFv antibody fragments were purified by anion exchange chromatography, The yields were 5-20 mg/L culture medium. The large-scale production of one scFv antibody fragment in a 3.7-L fermenter gave a yield of 60 mg, The reactivity of the cyteines was demonstrated by labeling with the thiol-reactive fluorescent dye ABD-F, The four scFv antibody fragments bound specifically to ED-B-modified Sepharose and binding was further confirmed by immunofluorescence on cell cultures using ED-B-positive human Caco-2 tumor cells. Furthermore, we could demonstrate specific binding of scFv-modified liposomes to ED-B-positive tumor cells. Our results indicate that the P, pastoris expression system is useful for the large-scale production of cysteine-functionalized alpha -ED-B scFv antibody fragments.