The authors present an original mechanism of the passive enhancement phenomenon which takes in account recent data, obtained in the rat and in the mouse, concerning specificities of enhancing Ig. Antibodies directed against "serological defined" antigens of the graft are not required for enhancement. An enhancing serum, deprived from anti "SD" antibodies still retains the ability to specifically block the target strain Fc receptor without any effect on lymphocyte harvested from strain carrying other major histocompatibility loci. The relationship between the Fc receptor of lymphocytes and Ia antigens are discussed since the enhancing serum deprived in anti-"SD" antibodies by absorption procedures could be referred as an anti-Ia serum. After having reviewed the data on the impairment of the immune response of enhanced host against the graft, an hypothesis involving graft lymphocytes Fc receptor blockage and abnormalities of the T-B lymphocytes cooperation in the enhanced animal is developped.