An scFv has been engineered to bind carcinoembryonic antigen (CEA) with a dissociation half-time >4 days at 37 degrees C. Two mutations responsible for this affinity increase were isolated by screening yeast surface-displayed mutant libraries by flow cytometry. Soluble expression of the mutant scFv in a yeast secretion system was increased 100-fold by screening mutant libraries for improved yeast surface display level. This scFv will be useful as a limiting case for evaluating the significance of affinity in tumor targeting to non-internalizing antigens.