SC142-reactive antigen are highly glycosylated glycoproteins expressed on tissues of gastric and colon cancers but not on normal tissues. Murine SC142 antibody specific for the SC142-reactive antigen has been produced by immunisation with SNU16 stomach cancer cells. However, SC142 antibody has several potential problems such as high immunogenicity and poor tumour penetration owing to their large size. To improve tumour penetration potential in vivo, recombinant single-chain fragments have been produced using the original hybridoma cells as a source of variable heavy-and variable light-chainencoding antibody genes. The use of the polymerase chain reaction, expression cloning technology and gene expression systems in E. coli has led to the production of SC142 single-chain fragments, which was similar in activity to the SC142 parent antibody confirmed by immunohistochemistry. Analysis by DNA sequencing, SDS -PAGE and Western blotting has demonstrated the integrity of the single-chain fragments. Competitive ELISA showed that SC142 single-chain fragments originated from parent SC142 antibody. BIAcore biosensor binding experiments showed that the SC142 single-chain fragments had an ideal dissociation rate constant as a tumour imaging reagent. These results illustrate the potential application of these novel products as an immunodiagnostic and further immunotherapeutic reagent. British Journal of Cancer (2002) Mucins are high-molecular-weight glycoproteins composed by a carbohydrate moiety (mainly O-glycans) that represents 50 -80% of their total mass and peptide core; also called apomucin, it is rich in Thr and Ser (Verma and Davidson, 1994). A variety of alterations of mucins have been described in metaplastic and malignant disease of the stomach (Correa, 1988). To detect those cancer associated alterations of gastric mucin, a number of monoclonal antibodies (mAbs) were developed. Although their specificity for cancer was limited compared with that of genetic markers such as p53, APC, c-met, k-sam and c-erbB-2, immunohistological detection of antigen is much easier than molecular biological analysis of those gene abnormalities, and this is of practical importance.In the previous study, we developed monoclonal antibodies elicited to the human stomach carcinoma cell line SNU16 to detect useful markers for gastric cancer. One of these monoclonal antibodies, SC142, detects an antigen present on adenocarcinoma cells of stomach. Preliminary studies on the molecular properties of the SC142-reactive antigen suggest that the epitope is sensitive to Oglycanase and is expressed on a large, heavily glycosylated molecule indicating that the antigen is probably mucin (Hong et al, 2001).In immunohistochemical studies, SC142-reactive antigen was not detected in normal gastrointestinal epithelium, whereas it was highly expressed in 78% of gastric cancers (29 out of 37) and 87% of colon cancers (27 out of 31) indicating that SC142 antibody can be a valuable tool to detect gastrointestinal cancers.However, the use of whole antibody fo...