SEVERAL research units have studied the genetic mechanisms controlling the synthesis of immunoglobulins and its alteration in hypogammaglobulinaemia. Fudenberg et al.1 investigated Gm and Inv groups in families with agammaglobulinaemia, and suggested the involvement of two genes in the synthesis of an immunoglobulin: one controlling the synthesis of the heavy chains, the other that of the light chains. Burtin et al.2 applied the same principle in their studies on “non sex-linked atypic hypogammaglobulinaemia”, and postulated the existence of abnormal alleles leading to an impaired synthesis of the heavy chains of one or several immunoglobulins.