Mer-NF5003F, a sesquiterpene isolated from Stachybotrys, inhibits avian medulloblastoma virus (AMV) protease (IC50=7.8 μM). Mer-NF5003F inhibits sialyltransferase 6N (ST6N), ST3O, and ST3N (IC50=0.61, 6.7, and 10 μg/mL, respectively), and fucosyltransferase (IC50=11.3 μg/mL). Mer-NF5003F has in vitro activity against herpes simplex virus HSV-1 (IC50=4.32 μg/mL) and multidrug-resistant Plasmodium falciparum K1 strain (IC50=0.85 μg/mL).