In the course of our screening to identify novel PPAR-gamma modulators for the potential treatment of type 2 diabetes, four new chlorinated angucyclinones, chlorocyclinones A-D ( 1- 4), were isolated from the mycelium of Streptomyces sp. strain DSM 17045. Their structures were established by spectroscopic methods. Chlorocyclinones antagonize rosiglitazone-induced peroxisome proliferator-activated receptor gamma (PPAR-gamma) activation with IC 50's < 0.4 microM in vitro using an AlphaScreen assay and are able to displace rosiglitazone from the PPAR-gamma ligand-binding domain (LBD) in a scintillation proximity assay (SPA). The compounds proved to be active in a cell-based reporter gene assay as well, antagonizing rosiglitazone-induced PPAR-gamma activity with IC 50 values between 0.60 and 7.0 microM. Chlorocyclinone C ( 3) exhibited the most potent activity in all assays.