Mer-NF5003F is a sesquiterpene originally isolated from Stachybotrys with diverse biological activities. It inhibits avian myeloblastosis virus (AMV) protease (IC = 7.8 µM). Mer-NF5003F inhibits sialyltransferase 6N (ST6N), ST3O, and ST3N (ICs = 0.61, 6.7, and 10 µg/ml, respectively), as well as fucosyltransferase (IC = 11.3 µg/ml). It is active against herpes simplex virus 1 (HSV-1) in vitro (IC = 4.32 µg/ml). Mer-NF5003F is also active against the multidrug-resistant P. falciparum strain K1 (IC = 0.85 µg/ml).