PURPOSE: To obtain the subject enhancer, containing an interleukin 6 antagonist and capable of enhancing antitumor actions of an antitumor agent, reducing the required dose of the antitumor agent and further raising the sensitivity of the antitumor agent even to tumors resistant to treatment.

CONSTITUTION: This enhancer contains an interleukin 6 (IL-6) antagonist. An IL-6R antibody such as PM-1 antibody is preferred as the IL-6 antagonist. The antibody is preferably obtained by carrying out a well-known method, etc., for using IL-6 as a sensitization antigen, immunizing an animal therewith, then fusing an immunized cell to a cell of a myeloma and subsequently producing the monoclonal antibody in a cell, screened from the fused cell and capable of producing the monoclonal antibody. Furthermore, the IL-6R which is the sensitization antigen is preferably derived from a human. Mitomycin C or a platinum compound such as cisplatin or carboplatin is preferred as an antitumor agent used in combination with the enhancer.