Phomopsis sp. FT-0211, a soil isolate, was found to produce inhibitors of lipid droplet formation in mouse peritoneal macrophages. Structurally related new compounds designated phenochalasins A and B were isolated from the fermentation broth of the producing strain by solvent extraction, ODS column chromatography and preparative HPLC. Phenochalasin A caused a dose-dependent reduction in the number and size of lipid droplets in macrophages without any cytotoxic effect at least up to 20 microm. On the other hand, phenochalasin B showed inhibition of lipid droplet formation with a severe cytotoxic effect on macrophages.