Ilicicolin H is a broad spectrum antifungal agent showing sub micro g/mL MICs against Candida spp., Aspergillus fumigatus and Cryptococcus spp. It is a potent inhibitor (C50 2-3ng/mL) of the mitochondrial cytochrome bc1 reductase with over 1000-fold selectivity against rat liver cytochrome bc1 reductase. Structure-activity relationship of semisynthetic derivatives by chemical modification of ilicicolin H and its 19-hydroxy derivative produced by biotransformation have been described. Basic 4'-esters and moderately polar N- and O-alkyl derivatives retained antifungal and the cytochrome bc1 reductase activities. 4',19-Diacetate and 19-cyclopropyl acetate retained antifungal and enzyme activity and selectivity with over 20-fold improvement of plasma protein binding.