IN 1961 Allison and Blumberg described genetically determined low-density β-lipoprotein differences in human sera revealed by means of a gel-diffusion technique, where about 60 per cent of U.S. white sera formed precipitates with serum from a repeatedly transfused patient (C. de B.)1-3. The new system was called the Ag-system and the two types of human sera revealed by the antiserum C. de B. were called Ag (a +) (reactors) and Ag (a -) (non-reactors). In 1963, Blumberg and Riddell described a second antiserum (the New York antiserum I. M.) reacting with about 97 per cent of U.S. white sera3. Sera forming precipitates with the I. M. antiserum were denoted as Ag (b +), non-reactors as Ag (b -)3. Recently, Bundschuh et al.4 described another anti-Ag serum reacting with about 65 per cent of sera from individuals living in Berlin (D.D.R.).