ELISA was applied for analysis of the HSA-human IgG autoantibody system responsible for the immunoelectrophoretic 'Tailing Albumin' (TA) phenomenon induced in most of the TA patients by prolonged nitrofurantoin therapy. Both hyperimmune porcine anti-HSA and autoimmune human anti-HSA antibodies of the IgG class were detectable by ELISA. The presence of autologous or added HSA had some inhibitory effect upon the detectability of the anti-HSA antibodies. Partial elimination of the autologous HSA by sucrose gradient ultracentrifugation or salt precipitation increased or unmasked the anti-HSA activity of some TA sera. The sensitivity of the ELISA as detector of the anti-HSA autoantibodies of whole human sera was roughly equal to that of the immunoelectrophoretic TA phenomenon. The analogy of the anti-HSA autoantibodies and the rheumatoid factors and the theoretical interest of both of them is stressed.