Anti-Sd^a is of no clinical significance, because it rarely causes hemolytic transfusion reactions. Even when its presence is suspected during antibody screening test, further identification of the antibody is usually not performed. We experienced a case of anti-Sd^a in 73 yr-old male patient showing mixed field agglutination by microcolumn agglutination. Antibody specificity could not be identified by conventional antibody identification test, and it was proven to be anti-Sd^a by urine neutralization test. In spite of its little clinical significance, it may give incompatible crossmatching results reacting with Sd^a antigen, which occurs at a high frequency in general population. When incompatible crossmatch results arising from anti-Sd^a are suspected, the problem may be solved by using the urine-neutralized serum of in crossmatching test.