Objective. Using monoclonal antibody (mAb) Fab′ fragment to develop mAb immunoconjugates for cancer. Methods. Fab′ fragment of mAb 3A5 was prepared by digestion of the antibody with pepsin and then reduced by dithiothreitol (DTT), while Fab′ fragment of mAb 3D6 was obtained by digestion of the antibody with ficin and subsequently reduced by β mercaptoethanol. The conjugation between Fab′ fragment and pingyangmycin (PYM), an antitumor antibiotic, was mediated by dextran T 40. Immunoreactivity of Fab′ PYM conjugates with cancer cells was determined by ELISA, and the cytotoxicity of those conjugates to cancer cells was determined by clonogenic assay. Antitumor effects of the Fab′ PYM conjugates were evaluated by subcutaneously transplanted tumors in mice. Results. The molecular weight of Fab′ fragment was approximately 53 kD, while the average molecular weight of Fab′ PYM conjugate was 170 kD. The Fab′ PYM conjugates showed immunoreactivity with antigen relevant cancer cells and selective cytotoxicity against target cells. Administered intravenously, Fab′ PYM conjugates were more effective against the growth of tumors in mice than free PYM and PYM conjugated with intact mAb. Conclusion. Fab′ PYM conjugate may be capable of targeting cancer cells and effectively inhibiting tumor growth, suggesting its therapeutic potential in cancer treatment. Fab. Fragment of mAb 3A5 was prepared by digestion of the antibody with pepsin and then reduced by dithiothreitol (DTT), while Fab ’fragment of mAb The conjugation between Fab ’fragment and pingyangmycin (PYM), an antitumor antibiotic, was mediated by dextran T 40. Immunoreactivity of Fab’ PYM conjugates with cancer cells was determined by ELISA, and the cytotoxicity of those conjugates to cancer cells was determined by clonogenic assay. Antitumor effects of the Fab ’PYM conjugates were evaluated by subcutaneously transplanted tumors in mice. Results. The molecular weight of Fab’ fragments was approximately 53 kD, while the average molecular weight of Fab ’PYM conjugate was 170 kD. The Fab’ PYM conjugates showed immunoreactivity with antigen rele vant cancer cells and selective cytotoxicity against target cells. Administered intravenously, Fab ’PYM conjugates were more effective against the growth of tumors in mice than free PYM and PYM conjugated with intact mAb. Conclusion. and effectively inhibill tumor growth, suggesting its therapeutic potential in cancer treatment.