Summary.A new private blood group antigen, SAT, was identified in an NFLD‐Japanese woman as a result of testing 10,480 blood donors with a serum containing anti‐NFLD and anti‐SAT. Three other sera were subsequently also shown to contain anti‐SAT. The donor's family showed that SAT is inherited as a dominant character and may be associated with a weak M antigen. Serological and immunochemical analysis revealed no other aberrations in the MNS system.Study of a second SAT+ Japanese blood donor and his family suggested that SAT is associated with an unusual MNS variant resulting from a hybrid glycophorin comprising the N‐terminus of glycophorin A and the C‐terminus of glycophorin B. The propositus appears to be homozygous for the gene that produces the putative hybrid, which differs from previously described glycophorin (A‐B) hybrids by expressing no S, s or U antigen. SAT antigen, therefore, may be associated with two different MNS variants in the only two families in which it has be