Systemic lupus erythematosus (SLE) is a multisystem autoimmune disease characterized by various types of immunological abnormalities including circulating and tissue-fixed autoantibodies reactive with autoantigens. The mechanism that can explain the production of these antibodies is unclear. Here we address the binding specificity of SLE autoantibodies with recombinant alpha interferon 2b (hrIFN α-2b), commercially available interferon (IFN α-2b), and the gene (cIFN α-2b) encoding this interferon. hrIFN α-2b showed higher binding with naturally occurring SLE autoantibodies as compared to IFN α-2b (p < 0.05) or cIFN α-2b gene (p < 0.001) as assessed by direct binding, inhibition ELISA, and quantitative precipitin titration. The relative affinity of SLE autoantibodies for hrIFN α-2b, IFN α-2b, and cIFN α-2b gene was in the order of 1.13 × 10(-7) , 1.38 × 10(-6) , and 1.22 × 10(-6) , respectively. hrIFN α-2b is shown to have unique epitopes that would explain the possible antigenic role of hrIFN α-2b in the generation of SLE autoantibodies. Anti-hrIFN α-2b antibodies have been shown to represent an alternative immunological probe for the estimation of interferon alpha 2b in the serum of SLE patients.