Galectin 9 exerts various functions depending on its localizations. On the other hand, galectin 9 is expected as participating in various biological functions. Thus, it has been required to clarify the detailed biological activities of galectin 9 and develop galectin 9-related techniques including development of drugs. Human galectin 9 shows a cytotoxic activity and an apoptosis-inducing activity on tumor cells but shows neither cytotoxic activity nor apoptosis-inducing activity on normal cells. Therefore, it is possible to employ galectin 9 proteins, galectin 9 agonists, galectin 9 antagonists, anti-galectin 9 binding protein antibodies, anti-galectin 9 binding sugar chain antibodies, galectin 9-producing, releasing or inducing substances, etc. as antitumor, antiallergic, immunosuppressive agents, drugs for autoimmune diseases, anti-inflammatory agents and active ingredients for adrenocortical steroid hormone alternatives.