In this study, we generated a new group of rabbit mAbs specific for previously undescribed epitopes in mesothelin, and showed that 1) the new antibodies such as YP218 and YP223 can be used to detect soluble mesothelin in the presence of Region I binders including SS1P, 2) they are suitable for immunohistochemistry, 3) the immunotoxin based on YP218 exhibits potent anti-tumor activity in mice bearing xenograft mesothelioma and has no severe in vivo toxicitiy, and 4) the rabbit YP218 Fv can be humanized by CDR grafting without loss of binding affinity for the antigen. This evidence concerns the gene MSLN and mesothelioma.