The interaction of 4-1BB and its ligand plays an important role in the regulation of T-cell-mediated immune responses. In this study, the authors examined the effect of a humanized anti--4-1BB monoclonal antibody (H4B4) on ovalbumin-induced immune responses in baboons. Previously, a mouse monoclonal antibody, 4B4 against the human 4-1BB molecule, was generated and characterized. Based on this antibody, a humanized version of 4B4 monoclonal antibody was constructed and the resultant antibody, H4B4, showed full recovery of the binding activity of the original antibody 4B4: a 1.5-fold increase in affinity for 4-1BB. In addition, H4B4 mediated antibody-dependent cellular cytotoxicity of activated human peripheral blood T cells and CEM cells in a dose-dependent manner. Weekly administration of H4B4 at doses of 1 or 4 mg/kg could suppress immunoglobulin G production against ovalbumin. This was not a result of the overall immune suppression, because the numbers of B and T cells and the total immunoglobulin G production were not altered during treatment with H4B4. These findings suggest that treatment with H4B4 may be a valid therapeutic approach to control unwanted immune responses in persons with autoimmune diseases.