Purity >98% by NMR, TLC. Geldanamycin-FITC is a novel geldanamycin fluorescent probe that may be used in a fluorescence polarization assay for HSP-90 inhibitors. May also be used for detection of cell surface HSP-90 and for other applications. Fluorescent geldanamycin probes have been used to characterize geldanamycin drug-target interactions for effective translation of in vitro pharmacology to cellular and in vivo efficacy.