The rare blood group phenotype lacking Lutheran antigens, Lu(a–b–), is known to have two genetic backgrounds. Tests on 250000 blood donors show the frequency of Lu(a–b–) to be approximately 1 in 3000. The families of 41 propositi show the dominant inhibitor of Lutheran antigens, In(Lu), to be the usual cause of the phenotype in South East England; there was no proven case of the recessive background, LuLu. Lod scores for In(Lu) and other blood group loci are presented; the only hint of linkage is between In(Lu) and Rh. The suppressing effect of In(Lu) on the expression of antigens of unrelated blood group systems, P1, Aua and i, is confirmed.