Monoclonal antibody IVA-285 (IgG1 isotype) recognizing antigenic determinant on bovine and ovine immunoglobulin light chain was produced and characterized. Western blot analysis of bovine immunoglobulin G (IgG) and immunoglobulin M (IgM) purified from bovine blood serum as well as whole immunoglobulin fractions of bovine and ovine serum with IVA-285 showed a molecular weight in the 24-27 kd range corresponding to the Ig light chain of bovine Ig. IVA-285 recognizes the Ig light chain on Ig+ lymphocyte subpopulation and in the majority of body fluids; however, especially strong reactions were observed in bovine tissues (lymph node follicles, plasma cells, and Ig deposits in various tissues).