In the screening for new melanin synthesis inhibitors, we found a novel isonitrile inhibitor, MR304A from the culture broth of Trichoderma harzianum. There were some isonitrile antibiotics that were reported to have been isolated from Trichoderma sp. 1 ~ 4) and trichoviridin, one of these isonitriles, showed a melanin biosynthesis inhibitory activity5). In this paper we described the fermentation, isolation, physico-chemical properties, structure determination and biological activities of MR304A.