Lambertellols A (1) and B (2), isolated from mycoparasites Lambertella species, were synthesized. The synthesis features intramolecular aldol-type cyclizations of aldehydes 12 and 14 and site specific oxidations of 1-hydroxylambertellols as key steps. The synthesis also provided all diastereomers of 1-hydrolambertellols 17-19 and 25. Chiral resolution made the optically active forms available, which enabled the investigation of the real active species in the mycoparasitism by Lambertella species against Monilinia fructigena. These experiments suggested that lambertellin (3) is responsible for this phenomenon. Chemically labile 1 and 2 should be converted to 3 during the bioassay. The parasite may excrete 1 and 2 as readily diffusible forms, which are then transformed into 3 to inhibit the host M. fructigena. The parasite may have acquired this "drug delivery system" mechanism as an evolutionary enhancement.