We validated expression and biological activities of plant-derived monoclonal antibody (MAb(P)) CO17-1A for its efficacy in cancer immunotherapy. PCR and immunoblot analyses demonstrated insertion and expression of heavy and light chains of MAb CO17-1A in transgenic plants, respectively. Confocal analysis revealed that MAb(P) CO17-1A was accumulated throughout the cytoplasm near the outer membrane, suggesting its secretion to the outer membrane via a default pathway. Cell ELISA analysis confirmed that the MAb(P) CO17-1A heavy and light chains in crude plant leaf samples assembled to specifically bind SW948 human colorectal carcinoma cells. Flow cytometry analysis showed that the Fc domains of both the purified MAb(P) and the mammalian-derived MAb (MAb(M)) evidenced similar binding activity to the FcgammaRI receptor (CD64). The biological activities of both MAbs were similar, although the glycosylation pattern of MAb(P) CO17-1A is distinct from that of MAb(M). These results point to the potential use of MAb(P) CO17-1A for colorectal cancer immunotherapy.