Two examples of a “new” IgM saline‐agglutinating auto‐antibody are described. The antibodies bind complement, have the ability to cause in vivo hemolysis, and are most active at room temperature at a pH of about 6.5. Despite tests on more than 5,000 people, no nonreactive cell sample has been found. The reactive antigen is not denatured by neuraminidase, papain, or ficin, and is present on i adult red blood cells. The antibodies appear to be slightly inhibited by human saliva and milk, and more convincingly inhibited by urine from Sd(a+) persons. They are not inhibited by urine from Sd(a‐) persons, but are strongly inhibited by guinea pig urine. The serologic characteristics indicate a relationship to the Sdablood group and the autoantibody has been named anti‐Sdx. Sdxantigen is present on red blood cells from some higher primates and is absent from rabbit, rhesus monkey, dog and