Five murine monoclonal antibodies, raised against E. coli derived human IL-4, were established. All mAb were also reactive with natural IL-4. Competition ELISA experiments revealed that mAb 1,2 and 4 recognized a related epitope on IL-4. mAb 5 and mAb 6 recognized another epitope. Two non-competing mAb were used to develop a sandwich IL-4 ELISA. mAb 5 was used for coating and biotinylated mAb I was used as the second antibody. Intra- and interassay-coefficients were 3.3 and 10.1% respectively. The ELISA is specific for IL-4, rapid and sensitive (the detection limit is 2 pg/ml). The capacities of the antibodies to inhibit IL-4 activity were tested in B cell and T cell assays. All antibodies inhibited IL-4 dependent IgE production by human B lymphocytes. A similar inhibition of IL-4 driven T cell proliferation by the antibodies was observed, with the exception that mAb 4 did not affect the activity of IL-4 on T cells. These results led to the suggestion that B cells make use of another (additional) IL-4 receptor chain.