Sporeamicins A (Antibiotic L53-18 A) and B are new 14-membered macrolide antibiotics isolated from Saccharopolyspora sp. L53-18, with their taxonomy of the producing culture, fermentation, isolation, structure determination and biological properties described in preceding communications. During the search for other antibiotics in the culture filtrates of the sporeamicin-producing strain, a new minor component designated sporeamicin C was discovered. This communication describes its fermentation (carried out at 28°C for 161 hours in a 250-liter fermenter containing a medium consisting of glucose 3%, corn steep liquor 1%, dry yeast 0.6%, cobalt chloride 0.001% and FS antifoam 028 0.04% (pH 7.0)), isolation and purification (accomplished using the general procedure for basic macrolide antibiotics, including extraction with ethyl acetate at pH 9.0, silica gel column chromatography and preparative reverse-phase HPLC (TSK gel 120T), with monitoring by bioautography using Micrococcus luteus ATCC9341 and HPLC analysis), physico-chemical properties (basic in nature, soluble in methanol, ethanol, acetone, ethyl acetate, benzene, chloroform and acidic water, virtually insoluble in n-hexane and water; positive color tests with potassium permanganate, iodine, Dragendorff and Molisch reagents, unreactive with ninhydrin or Sakaguchi reagent; molecular formula C36H61NO12 determined by FAB-MS (m/z 700, (M+H)+) and elemental analysis; UV spectrum suggesting an enone function (276 nm); IR spectrum showing enone (1620, 1690 cm⁻¹), ester carbonyl (1740 cm⁻¹) and hydroxyl (3450 cm⁻¹) functions), and structure determination (based on CI-MS and NMR data, indicating it has one less CH2 atom than sporeamicin A, shares the same aglycon moiety as sporeamicin A, and the substituted position on the desosamine moiety was determined by NMR experiments).