Thiol derivative, a competitive hypoxanthine antagonist, has an immunosuppressive and cytostatic effect. It is used for prevention of rejection reaction in kidney transplantation, treatment of rheumatoid arthritis, chronic active hepatitis, SLE, dermatomyositis, periarteritis nodosa, acquired hemolytic anemia, gangrenous pyoderma, psoriasis, Crohn's disease, ulcerative colitis and myopathy.