Beside sorangicin, disorazol, and chivosazol, we found a fourth biologically active substance with a new structure in the XAD extract of the fermentation broth of Sorangium cellulosum strain So ce 12, proposing the name sorangiolid A. Here we report its biological activity (isolation and structure elucidation described elsewhere). Sorangiolid A was active against Gram-positive bacteria (MIC 5–20 μg/ml), insensitive to yeasts and fungi, but sensitive to mammalian cells. It simultaneously and immediately inhibited the synthesis of various macromolecules (DNA, RNA, protein, cell wall) in Staphylococcus aureus, suggesting interference with fundamental cellular structure or biochemical reaction (e.g., energy metabolism or membrane integrity). It increased the permeability of S. aureus and sheep erythrocytes, leading to efflux of UV-absorbing material. The effect on S. aureus was bactericidal: viable cells decreased rapidly within 2 hours (to ~1% in nutrient broth, ~0.01% in Tris-HCl buffer). A minor structural variant sorangiolid B (additional OH-group in the side chain) was present. MIC depended on cell density (2.5 μg/ml for 10³ cells/ml, 5 μg/ml for 10⁵, 10 μg/ml for 10⁷). Preincubating the broth with 10⁹ cells/ml and removing cells by centrifugation increased the MIC in the supernatant by a factor of 3, indicating binding to cells or cell debris. The strong bactericidal action is likely due to membrane destruction, which also induced hemolysis in sheep erythrocytes. Surviving cells after 2 hours were not resistant, possibly due to sorangiolid A removal by binding.