An anti-KC-4 humanized monoclonal antibody that comprises the variable regions of the light and heavy chains of the anti-KC-4 murine antibody, wherein the light chain has 7 amino acids and the heavy chain has 12 amino acids of the framework regions substituted with amino acid present in equivalent positions in antibodies of a species other than munne, and the constant regions of a human antibody. The antibody may be labeled and/or glycosylated, and is presented as a composition with a carrier. The anti-KC-4 monoclonal antibody is used in diagnostic kits for cancer and in in vivo methods of imaging and treating a primary or metastasized cancer, and in vitro diagnosis and ex vivo purging neoplastic cells from a biological fluid. RNAs and DNAs encode the monoclonal antibody, and a hybrid vector carrying the nucleotides and transfected cells express the peptides.