The effects of FK506 (tacrolimus hydrate) ointment on cutaneous allergic reactions in mice and rats were investigated. FK506 ointment showed significant suppressive effects on delayed allergic reactions in both species, and especially in rats, its inhibitory action was much stronger than that of alclometasone dipropionate, a so-called medium class steroid ointment. On the other hand, FK506 ointment did not inhibit the immediate allergic reaction in rats. FK506 ointment suppressed the delayed allergic reactions in locally passively sensitized mice to the same degree as that in actively sensitized mice. Accordingly, it is speculated that FK506 ointment inhibits the activation of sensitized T lymphocytes (Th1 cells) already accumulated in the dermis.