Abstract: An objective of the present invention is to provide NKT cell-activating agents, therapeutic agents for autoimmune diseases (for example, systemic lupus erythematosus, systemic sclerosis, ulcerative colitis, encephalomyelitis, multiple sclerosis and human type I diabetes), and abortifacients. The medicinal compositions according to the present invention comprise &agr;-glycosylceramides of the following formula (I), or a salt or a solvate thereof as an active ingredient.