The present invention provides an antibody comprising an antigen binding domain which binds to human IL-12 and human IL-23. The antibody binds human IL-12p40 existing as a monomer (human IL-12p40) and as a homodimer (human IL-12p80) and the antibody inhibits the binding of human IL-12 to human IL-12R beta2 and human IL-23 to human IL-23R but does not inhibit the binding of human IL-12 or human IL-23 or human IL-12p40 or human IL-12p80 to human IL-12Rbeta1.