An immunosuppressive chemical produced by Penicilium urticae (KFCC 11611) was purified from the supernatant of a culture broth by ethyl acetate extraction, silica gel column chromatography, and precipitation by adding cold n-hexane followed by crystalization. The chemical effectively inhibited the mouse mixed lymphocyte reaction (MLR) at a concentration of 1.25ⅹ10^3 ㎍/ml, which was lower than the effective concentration of cyclosporin A. The chemical was identified as the antibiotic patulin (MW 154.1 Da) by NMR and mass spectroscopy.