A new low incidence blood group antigen "Mit", immunization to which may cause hemolytic disease of the newborn, is described. It is not part of the ABO, Duffy, Kidd, Rh or Yt blood group systems. Positive lods of +2.41 at O = 0.00 indicate that the odds are 256 to 1 that "Mit" is either part of the MNSs system or governed by a locus closely linked to MN and Ss.