Geldanamycin, an antimicrobial compound, was purified in 1970 from the culture filtrates of Streptomyces hygroscopicus var. geldanus var. nova. Geldanamycin is moderately active in vitro against protozoa, bacteria, and fungi. It also has an antineoplastic effect against some cancer cells growing in culture. Molecular studies revealed the binding of geldanamycin to members of the heat shock protein 90 (HSP90) family of molecular chaperones. Interference with the function of these HSPs seems to be the major mechanism of action of geldanamycin.