AbstractUsing a panel of monoclonal antibodies which detect framework determinants of human‐Ia à and β polypeptides and the DC‐specific monoclonal antibody Genox 353, thein vitroeffects of 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) on the expression of Ia antigens by cells from chronic lymphocytic leukemia (CLL) patients have been studied. In all subjects studied a great increase in expression of framework Ia determinants was found following culture of CLL cells for 90 h with TPA at 10−7g/ml. TPA treatment of CLL cells also induced increased expression of Genox 353+antigens, in some cases where Genox 353 binding was either negligible or not detectable in cells cultured in the