Abstract: This invention provides for recombinant single chain antibodies capable of specifically binding to a Lewis.sup.Y -related carbohydrate antigen and fusion proteins comprising these antibodies. More particularly, the invention provides for single chain Fv regions of the monoclonal antibodies B1 and B5. The invention also provides for a number of stabilizing mutations of the Lewis.sup.Y -binding monoclonal antibody B3. In addition, the invention provides for methods of detecting cells bearing a Lewis.sup.Y antigen in a patient and for methods of killing or inhibiting the growth of cells bearing a Lewis.sup.Y antigen in a patient.