In the serum of a patient (E.Y.) who had received multiple blood transfusions, a precipitating antibody to a component in human serum was found. The antigen revealed by this antibody was demonstrated in the serum of one out of 222 healthy blood donors. The E.Y. antigen apparently belongs to a component of high molecular weight. A small amount of lipid may be associated with the antigen. The E.Y. antigen is apparently related, and possibly identical, to the SH antigen described by Prince (1968a).