Radicicol (RAD) and the benzoquinone ansamycin geldanamycin (GA) are potential anticancer drugs known to inhibit heat shock protein 90 (hsp90) and, therefore, the activation of proteins dependent on its function such as proto-oncogenic kinases and nuclear receptors. Using the glucocorticoid receptor (GR) as a model system we analysed the effects of RAD and various benzoquinone ansamycins. All compounds efficiently abolished GR-dependent transactivation. Surprisingly, whenever one of the ansamycins was applied in combination with RAD, synergistic inhibition of GR-dependent transcription and of hormone binding of GR was observed. In contrast, combination of two ansamycins showed no synergy. These findings suggest synergism within the hsp90 dimer and may open new ways to explore hsp90 as therapeutic target.