Immunoblots of red cell membranes stained with eluates of alloanti-Aua and alloanti-Aub show that these antibodies recognize 2 membrane components from Au(a+) and Au(b+) cells, respectively. These structures, of apparent molecular weight (Mr) 79,000 and 85,000, are identical in appearance and mobility on a 10% SDS polyacrylamide gel to the Lutheran glycoproteins identified by alloanti-Lub. Like the Lutheran glycoproteins, they showed a reduction in apparent Mr of about 1,500 after sialidase treatment. Red cell membrane components immunoprecipitated by a Lutheran-related monoclonal antibody, were analysed with Lutheran and Auberger antibodies by immunoblotting. The Lutheran glycoproteins were revealed by anti-Lub in precipitates from Au(a+b-) Lu(a-b+) and Au(a-b+) Lu(a-b+) cells, by anti-Aua in precipitates from Au(a+b-) Lu(a-b+) cells and by anti-Aub in precipitates from Au(a-b+) Lu(a-b+) cells. The same components were also recognized by anti-Lua in precipitates from Lu(a+b-) cells. Thus Aua and Aub antigens appear to be carried on the same red cell membrane structures as those carrying the Lutheran determinants. These results are particularly significant in the light of the very close phenotypic association between the Auberger and Lutheran blood groups which have been shown, by one family, to be controlled by genes at separate loci.