Abstract: The invention provides for the production of several humanized murine antibodies specific for the antigen Lewis Y, which is recognized by murine atibodies specific for the Lewis Y antigen. The Lewis Y antigen is expressed in normal tissues but the level of expression is higher in certain tumour types so that the antigen can be used as a marker for cells of some breast, colon, gastric, esophageal, pancreatic, duodenal, lung, bladder and renal carcinomas and gastric and islet cell neuroendocrine tumours. The invention also provides for numerous polynucleotide encoding humanized Lewis Y specific antibodies, expression vectors for producing humanized Lewis Y specific antibodies, and host cells for the recombinant production of the humanized antibodies. The invention also provides methods for detecting cancerous cells (in vitro and in vivo) using humanized Lewis Y specific antibodies. Additionally, the invention provides methods of treating cancer using humanized Lewis Y specific antibodies.