Fifteen murine hybridoma lines that produce monoclonal antibodies against human interleukin-10 (IL-10), which is one of the most important regulatory cytokines of the immune system, were raised. Twelve of these antibodies, all in the class IgG1/kappa, recognize three groups of epitopes: A, B and C. All antibodies in these groups inhibit binding of antibodies of the same group to IL-10 and none of them inhibit binding of an antibody of another group. Two IgM/kappa antibodies and one IgG1/kappa antibody, with low affinity, have distinct binding properties and cannot be assigned to one of these groups. Antibodies from all three epitope groups inhibit the biological activity of IL-10. The three antibodies of group A neutralize IL-10 in an approximately equimolar ratio, at concentrations as low as 10 pM. In addition to their high neutralizing capacity, antibodies of group A enhance the binding of antibodies of group C to IL-10. The on-rate of binding of the two antibodies CB/RS/10 and 11 (group C) increased five- to seven-fold, when one of the antibodies CB/RS/1, 2 or 14 (group A) is bound to IL-10.