A low frequency red cell antigen, SHIN, was detected in two Japanese blood donors. Red cells of the first propositus reacted, in pretransfusion tests, with the serum of a previously untransfused male patient. The second propositus was disclosed when anti-SHIN was used to screen red cells from 3,000 random blood donors in Tokyo: a frequency of 0.03% in this population. Family studies showed SHIN to be inherited as an autosomal dominant character and not to be part of the MNS, FY, JK or P1 blood group systems. Anti-SHIN is not a common antibody in normal sera but was found in about 10% of multispecific sera used in investigation of low frequency antigens.