Abstract: A class of 2,6-diarylpyridazinones of general structural formula I have been identified that exhibit exhibit immunosuppressant activity with human T-lymphocytes, and are useful as an immunosuppressants. ##STR1## or a pharmaceutically acceptable salt, hydrate or crystal form thereof, wherein:when M is S, R.sup.1 and R.sup.2 are selected from the following combinations:when R.sup.2 is 4-chloro, then R.sup.1 is 4-OCH.sub.3,2 -CH.sub.3, 4-Cl, 4-CH3,3-Cl, 3-CH3, 2-Cl, 4-H, 4-Br, 3-NO.sub.2 ; andwhen R.sup.2 is H, then R.sup.1 is 4-OCH.sub.3, andwhen M is --SO.sub.2 --, then R.sup.2 is H and R.sup.1 is 4-OCH.sub.3.As an immunosuppressant, these compounds are useful in the treatment of autoimmune diseases, the prevention of rejection of foreign organ transplants and/or related afflictions, diseases and illnesses.