A monoclonal antibody and an affinity purified polyclonal antibody, both raised against recombinant human IL-6, have been employed in an ELISA procedure to quantitate human IL-6. Both antibodies were very potent in neutralizing the biological activity of recombinant as well as natural human IL-6. The monoclonal antibody was used as the capture antibody whilst the polyclonal antibody, in biotinylated form, was used as the detecting antibody in combination with a streptavidin horseradish peroxidase conjugate and a signal amplification system. The detection limit for natural as well as recombinant IL-6 was 1 pg/ml. A good correlation was found between the ELISA and the B9 biological assay when IL-6 was measured in crude culture supernatants, in synovial fluids of rheumatoid arthritis patients and in the sera of patients with diverse diseases. Immunoprecipitation of IL-6, produced by different cell types, such as monocytes, endothelial cells and smooth muscle cells or derived from biological fluids, such as the serum of a patient with septic shock or the synovial fluid of a rheumatoid arthritis patient, revealed in every case only molecules in the molecular weight range of 21,000-26,000.