A new crystalline antimicrobial compound, geldanamycin, has been discovered in the culture nitrates of Streptomyces hygroscopicus var. geldanus var. nova. Geldanamycin is moderately active in vitro against protozoa, bacteria and fungi. It is also active against L-1210 and KB cells growing in culture and against the parasite Syphacia ohlevata, in vivo. The fermentation, assay, chromatography and isolation as well as its biological and chemical properties were investigated. On the basis of its physical and chemical properties, geldanamycin is a complex molecule consisting of an unsaturated moiety attached to a quinone.