Four phenotypes of Cad (Cad 1–4) have been characterized by a continuum of polyagglutinability and reactivity with lectins, with the strongest Cad+ red blood cells (RBCs) being polyagglutinable because of the presence of anti-Cad (anti-sda) in most normal sera. Over a period of 7 years, a French male blood donor’s RBCs demonstrated polyagglutinability with 50 percent to 70 percent of normal adult sera. The reactivity was characteristic of anti-sda (refractile agglutination at 4°C, 20°C, 37°C, and anti-human globulin test), and was inhibitable by two examples of Sd(a+) urine, but not by Sd(a–) urine or dialysate from Sd(a+) urine. The donor’s RBCs reacted 1+ with