Genetic studies of the families of two unrelated propositi prove that the low-incidence antigen ELO (ISBT No. 700.51) is not part of the MNS, Rh, Duffy, Kidd, Xg, Kx and Gerbich blood group systems and indicate that it is probably not part of the Lutheran, Scianna, Dombrock or Colton systems. Serological studies indicate exclusion from the Kell and Chido/Rodgers systems. Although the ELO antigen is rare, anti-ELO is not uncommon in sera containing at least one specificity for a low-incidence antigen.